Guide · Raid Bosses

Ulduar 25 Hard Mode Boss by Boss

Ulduar 25 hard mode boss by boss: how to trigger every hard mode, the achievement tricks, and what each role does, from Flame Leviathan to Algalon.

By Ilia Kozarev (RealNibba) · Updated 22 Sep 2026

14 bosses5 areas25-player hard modesGlory of the Ulduar Raider

Ulduar is the raid where almost every boss has a hard mode: a harder version you switch on yourself, usually by doing something extra before or during the pull rather than by a gear check. This page covers all fourteen encounters on 25-player hard mode, with how each one is triggered, the mechanics that come with it, and what every role does. With a well-geared raid these fights are fast, so the real skill is remembering the trigger and not fumbling the one or two tricks each boss has.

Every hard mode trigger below is the one Blizzard built into the encounter, not a server-specific rule, so it holds up wherever you raid it. Your raid leader's own calls and assignments always come first. Hover or tap any coloured spell for its tooltip.

1.The Formation Grounds

Bosses 1 to 4

Flame Leviathan

The Formation Grounds

Vehicle fightHard mode: activate before the pull

The only vehicle fight in the raid. Talk to the Lore Keeper of Norgannon before you talk to Brann to start on hard mode, which arms Leviathan with three extra towers (Freya, Hodir, Thorim) that add mechanics on top of the normal fight. The raid splits into three vehicle types: Demolishers do the damage, Siege Engines interrupt and tank, and Choppers slow the boss and lay tar.

Mechanics

DamageDemolishers

Each one fires Pyrite at the boss for heavy damage plus a stacking burn; the goal is to keep the stack high the whole fight. When targeted by Pursuit, back away rather than turn, so the burn keeps ticking.

TankSiege Engines

They tank the boss up close and interrupt Flame Vents. Watch your steam gauge so the interrupt and the escape ability are both ready when needed.

SetupChoppers and tar

Choppers keep a patch of tar under the boss all fight, and on hard mode each one also covers a corner of the room, ready to ignite the plants that spawn there.

HeroicThe three towers

Freya's plants spawn in the corners (burn them in the tar), Hodir's blue lights chase and freeze anyone caught standing still, and Thorim's light beams hit hard, so keep moving and use defensives under them.

Who does what

Everyone

Everyone drives a vehicle. Spread the Demolishers and Siege Engines out at the start so a chased vehicle has room to kite, and let the Choppers work the corners once the towers are up.

Razorscale

The Formation Grounds

2 to 3 tanksAir phase + ground phaseHard mode: keep every Guardian alive

An air phase where dwarf adds spawn in waves, and a ground phase once she is harpooned down. The hard mode achievement asks you to have Dark Rune Guardians alive when she finally lands, so her own Flame Breath kills them for you instead of your raid killing them early.

Mechanics

AddDark Rune Guardians

Off-tanks pick up every Guardian that spawns and simply hold them without letting them die, kiting them around while healers keep the tanks topped. A low one can be sent to a Mage for Polymorph to buy it time.

TankSentinels and Watchers

The main tank holds these and the general add pressure while the Guardians are being collected.

HeroicWaiting for the enrage

Once her enrage timer hits, harpoon her to the ground. Her Flame Breath then kills the stockpiled Guardians in one pass, which is what the achievement is checking for.

Raid damageGround phase

On the ground she adds a fire wave and a wing buffet on top of the fight, so keep dodging and healing normally once she is down.

Who does what

Tanks

One tank on the Sentinel and Watchers, one or two off-tanks collecting and holding every Guardian without letting any die.

Healers

Keep the off-tanks topped through the whole collection phase; the tank damage grows as more Guardians pile up.

Damage dealers

Clear the smaller adds and keep the pressure on so the enrage timer arrives on schedule.

XT-002 Deconstructor

The Formation Grounds

1 tankStack in 3 groupsHard mode: kill the Heart at 75%

Hard mode turns on the moment you kill his exposed Heart the first time it appears, so the raid stacks in three tight groups (tank, melee, ranged) to make that first burst clean, and to keep the fight simple afterward.

Mechanics

SetupThe Heart

At 75% health his Heart exposes. Burn it down without cooldowns the first time: that kill is the hard mode switch, and the boss then resets to full health with new, harder abilities and no more adds.

MoveGravity Bomb

The marked player walks away from the raid and waits it out; on hard mode it also drops a void zone when it pops, so move off that spot immediately.

Raid damageTympanic Tantrum

Raid-wide damage every second for several seconds. Use a raid cooldown (Divine Sacrifice plus Aura Mastery, or a healing cooldown) for each one.

AddPummelers and Scrapbots

Before the Heart dies, a Pummeler gets misdirected onto the tank and cleaved down with the Heart; Scrapbots and Boom Bots are picked off by a ranged DPS before they reach the boss.

Who does what

Tanks

Hold the boss through the reconstruction, and pick up any Pummeler that spawns.

Healers

Save a raid cooldown for each Tympanic Tantrum.

Damage dealers

Do not use damage cooldowns before the Heart kill. Once the boss reconstructs, pop everything and burn him down.

Ignis the Furnace Master

The Formation Grounds

1 tank + off-tankUnder 4 minutesSide goal: Shattered

The hard mode itself just asks for a fast kill, which a geared raid does without trying. The interesting part is the optional Shattered side-achievement: the off-tank walks his Iron Constructs through fire until they are Molten, then into the water to make them Brittle, and ranged damage shatters them before the boss dies.

Mechanics

MoveScorch

A frontal cone that also scorches the ground for a while. Stay out of both the cone and the scorched patch.

AddIron Constructs

The off-tank drags each one through the scorched ground to stack Heat, then into the water once it turns Molten to make it Brittle and stunned. Ranged damage finishes it off there to shatter it, then everyone backs away from the shatter's splash damage.

TankMain tank

Holds Ignis away from the fire and Constructs so the raid keeps clean uptime.

Who does what

Tanks

Main tank holds the boss; off-tank walks the Constructs from fire to water.

Damage dealers

Stay out of Scorch, and land a solid hit on each Construct the moment it is Brittle to shatter it.

★

After the kill

This is the fastest boss to loop back through on the way to Auriaya, so clear the guards on the way and keep moving: your raid-wide cooldowns will be up again by the next pull.

2.The Antechamber

Bosses 5 to 7

Kologarn

The Antechamber

2 tanksHard mode: Disarmed

The hard mode achievement, Disarmed, asks you to destroy both of his arms and the body within 12 seconds of each other. The trick is timing: bring the body down first, then swap everyone onto both arms together so all three die almost at once.

Mechanics

DeadlyFocused Eyebeam

Two beams follow the targeted player. Run them away from the raid until they fall off.

DeadlyStone Grip

The right arm grabs three players; if it holds them for 15 seconds they die. Damage on that arm frees them, so switch to it the moment it grips someone.

SetupThe 35% swap

Focus the body only until it reaches 35% health, then split the raid onto both arms at once. Each arm kills deals 15% of the body's health, so killing both together finishes the fight instantly.

AddRumbles

Spawn once an arm dies. They hit hard on aggro, so keep a cooldown or an off-tank ready if your other arm is lagging behind.

Who does what

Tanks

Both tanks stand two steps ahead of the raid; no charge or leap abilities toward the boss, since walking in front of him is lethal.

Damage dealers

Body damage only until 35%, then split cleanly onto both arms and burn them down together.

The Iron Council

The Antechamber

2 tanks3 bosses, 1 poolHard mode: kill Steelbreaker last

Three bosses that share nothing but a room. Hard mode only requires that Steelbreaker dies last; killing Brundir first, then Molgeim, keeps the fight simplest, since Brundir is the most dangerous to leave alive and Molgeim leaves behind a damage buff worth standing in.

Mechanics

InterruptBrundir: Chain Lightning and Overload

Chain Lightning is interrupted every cast. Overload is a raid-wide burst he casts himself; if he lives long enough to get it off, spread and heal through it.

Raid damageMolgeim: Rune of Power and Rune of Death

Rune of Power is a buffed patch worth standing in for extra damage. Rune of Death lands under a random player, so if you are stacked on the power rune, step out a few seconds before the death rune is due and come back after.

TankSteelbreaker

A separate tank keeps him apart from the other two so their combined threat and abilities do not stack. Dispel Fusion Punch off the tank immediately, and swap tanks before Overwhelming Power stacks reach the point where Meltdown kills the holder.

HeroicAfter Brundir dies

The remaining two bosses gain a stacking enrage, so keep the pace up once he is down instead of slowing down.

Who does what

Tanks

One tank on Molgeim/Brundir, a second tank solely on Steelbreaker.

Damage dealers

Kill Brundir, then Molgeim while standing in his power rune, then finish on Steelbreaker.

★

Mages: spellsteal

Molgeim casts a damage-reduction shield on himself. A Mage who spellsteals it removes the shield from the boss and gets a personal shield in return.

Auriaya

The Antechamber

1 main tank + 2 off-tanksHard mode: Crazy Cat Lady

The hard mode achievement asks you to kill her without killing her four Sanctum Sentries (the cats). The whole pull is built around controlling those cats cleanly: two hunter traps pull them onto the main tank, then two off-tanks peel two cats each so nothing dies by accident.

Mechanics

SetupThe pull

Two Hunters trap the cats as Auriaya nears the platform, then misdirect them onto the tank who eats the first hit with a cooldown up. Nobody casts anything before the cats reach him, or the aggro can jump.

TankSplitting the cats

The moment the cats land on the main tank, two off-tanks peel two cats each with a minor defensive up, while the main tank keeps Auriaya.

DeadlyTerrifying Screech and Sentinel Blast

The fear is dispelled immediately since it can throw people out of position. Sentinel Blast follows it and must be interrupted, ideally by whichever tank has an interrupt with a short cooldown.

AddGuardian Swarm

Ten low-health adds swarm a marked player and tend to run to your healers. AoE them down the moment they appear.

Who does what

Tanks

Main tank keeps his back to the raid so Sonic Screech splits its damage across everyone instead of one person.

Healers

Two Holy Paladins or an equivalent spread of strong single-target healing handles the burst when the cats land.

Damage dealers

Kill nothing but Guardian Swarm adds; every cat must survive the whole fight.

3.The Keepers

Bosses 8 to 11

Hodir

The Keepers

1 tankHard mode: under 3 minutesSide goals: two achievements

Hard mode is a pure speed check: kill him inside three minutes and it is done automatically. Since a geared raid clears that easily, most groups use the spare time for two side achievements instead: never taking more than a couple of stacks of Biting Cold, and never letting anyone freeze during his one Flash Freeze cast.

Mechanics

Raid damageBiting Cold

Constant frost damage that only movement reduces. Keep moving or jumping the entire fight rather than standing still.

DeadlyFreeze

Roots and damages anyone within range. Dispel it, or break it with a shapeshift, Blink, or similar if you have one.

HeroicFlash Freeze

Long cast, three marked circles. Stand near a circle, and once the falling icicle turns it into a snowdrift, hop onto the drift and stay there until the freeze passes: standing in a snowdrift is what keeps everyone safe and the achievement clean.

SetupThe frozen helpers

Free the trapped NPCs early: the mage's bonfire and the druid's light patch both help the raid, and a shaman buff spreads to nearby players.

Who does what

Tanks

One tank, ideally with strong self-sustain, since there is no tank swap in this fight.

Everyone

Never stand still for more than a second or two, and get into a snowdrift the instant Flash Freeze starts.

Thorim

The Keepers

1 tankGauntlet + arenaHard mode: reach him fast

Hard mode is decided entirely by how quickly your gauntlet team fights through the tunnel and forces him down into the arena; a fast run keeps Sif in the fight and removes a damage-reducing buff from Thorim, which is exactly what you want for a quick, clean kill.

Mechanics

SetupTwo teams

A small, mobile gauntlet team (one tank, one healer, a few DPS) fights through the tunnel while the rest of the raid holds the arena and keeps an add mind-controlled to preserve a raid-wide speed buff until the gauntlet team arrives.

MoveThe Runic Colossus and Rune Giant

The Colossus telegraphs which side it will smash, so the gauntlet team moves together to the safe side. The Rune Giant's detonation debuff needs space around the marked player.

Raid damageChain Lightning (arena phase)

Bounces between anyone standing close together. Once the gauntlet team drops into the arena, everybody holds position rather than drifting into each other.

Raid damageLightning Charge

A telegraphed cone across part of the arena. Watch for the warning and step clear of it.

Who does what

Tanks

One tank takes Thorim in the arena; no swap is needed against a well-geared raid.

Damage dealers

The gauntlet group prioritizes speed over safety margins; the arena group saves burst for once the boss is down and everyone is settled.

Freya

The Keepers

1 tank3 EldersHard mode: leave the Elders alive

Each of her three Elders left alive hands Freya an extra ability, so the "safe" way to fight her is to kill them first. A well-geared raid instead leaves all three up and burns Freya down with cooldowns before the extra mechanics matter, which also happens to be the fast route through the fight.

Mechanics

Raid damageUnstable Energy (Brightleaf)

Beams of light that damage anyone standing under them. Step out as soon as they appear.

InterruptGround Tremor (Stonebark)

A short raid-wide cast that also interrupts casters. Use a raid cooldown for the damage and expect your own casts to be cut off.

AddIron Roots (Ironbranch)

Roots that grab and drain nearby players. Damage the root itself to break free, or simply avoid standing near one.

HeroicBurning her down with all three Elders alive

This is the deliberate hard-mode route: spread out, apply a damage-reduction debuff (an Expose Armor or Shattering Throw works well), and use every cooldown at once so the fight ends before the Elders' mechanics pile up.

Who does what

Tanks

A single strong tank holds Freya in the middle while the raid burns her down.

Damage dealers

Ignore every add that spawns from the Elders unless it is directly threatening someone; the only real target is Freya herself.

Mimiron

The Keepers

1 tank4 phasesHard mode: press the Red Button

Four short phases that test movement and positioning more than raw damage. Hard mode starts the moment someone presses the Red Button in his room, so make sure that is deliberate. The four phases are really four different fights back to back: the tank-and-spank leviathan body, the flying VX-001, the airborne command unit, and finally all three fused together as Voltron.

The phases

Phase 1Leviathan Mk II

Tank-and-spank with two dangerous single-target casts: Plasma Blast (cover with a defensive) and Shockblast (everyone breaks 15 yards away before it lands). Stop damage at 10% and let the flames clear before finishing him.

Phase 2VX-001

An untanked burst phase. Dodge the marked Rocket Strike and, if it happens, stand behind the boss and rotate with the slow-turning Laser Barrage.

Phase 3Aerial Command Unit

Ranged focus the flying unit while melee clean up Bomb Bots and Assault Bots. A Magnetic Core dropped under the unit pulls it to the ground for bonus damage.

Phase 4Voltron (all three fused)

Split the raid into three damage groups, one per fused part, and kill all three together so they cannot revive each other. All the earlier phases' mechanics can still appear.

Who does what

Tanks

One tank throughout, repositioning the boss away from incoming Frost Bomb zones between phases.

Damage dealers

Save cooldowns for phase 2 and phase 4, the two windows where raw damage matters most.

Positioning between phases

Frost Bomb zones and fire patches are placed in predictable spots between phases. Keep the raid together at the wall during the short downtimes so nobody gets caught out alone when the next phase opens.

4.The Prison of Yogg-Saron

Bosses 12 and 13

General Vezax

The Prison of Yogg-Saron

1 tankMana is the enemyHard mode: let all 6 Vapors merge

Hard mode is triggered by patience: instead of killing the Saronite Vapors that spawn, you let all six of them merge into a Saronite Animus, a second boss that must be burned down before Vezax can take damage again. The whole fight is also fought under a debuff that blocks most mana regeneration, so healers lean on whatever cooldowns are exempt from it.

Mechanics

Raid damageShadow Crash

Lands near a random ranged player and leaves a puddle that boosts magic damage and casting speed at the cost of healing done. Ranged DPS stand in it, healers avoid it.

InterruptSearing Flames

An armor-shredding cast that must be interrupted every time. Assign your strongest interrupters to a rotation for it.

DeadlyMark of the Faceless

Drains and heals the boss from nearby players. Whoever is marked spreads well away from anyone else before it expires.

HeroicSaronite Animus

Once all six Vapors merge, burn the Animus down immediately with every cooldown you have; it hits the raid with a stacking shadow debuff the whole time it is alive, and Vezax cannot be damaged again until it dies.

Who does what

Tanks

A tank with strong single-target self-healing, since Vezax cannot be taunted and threat management is everyone's own job.

Healers

Lean on the mana tools that bypass the fight's mana-regen debuff, and keep enough in reserve for the Animus burn.

Damage dealers

Never attack a Vapor. Let all six merge, then dump every cooldown into the Animus the moment it appears.

Yogg-Saron

The Prison of Yogg-Saron

1 to 2 tanks3 phasesHard mode: no Keepers summoned

Hard mode simply means walking straight past every Keeper statue without asking for help, which also removes the Sanity-restoring wells from the room. Sanity is the resource this entire fight is built around: let it run out and you are mind-controlled against your own raid for the rest of the pull.

The phases

Phase 1Sara and the Guardians

The raid stacks near Sara and kills Guardians as they spawn or wander into the moving light clouds. Interrupt each Guardian's Dark Volley before it hits the raid.

Phase 2Tentacles and the Brain

Melee ignore Crusher Tentacles (hitting them only makes them worse) and clean up Corruptor Tentacles instead. A dedicated small team enters the brain portals to clear each vision room and burn the exposed brain.

Phase 3The final burn

Yogg is rooted in place with no threat table, so it is a clean burn. Face away whenever Lunatic Gaze is about to fire, and use a silence-resistance cooldown for Deafening Roar.

Mechanics

AddConstrictor Tentacles

Grab and squeeze a random player. Kill them fast to free the target.

DeadlyMalady of the Mind and Brain Link

A fear that drains Sanity and jumps between nearby players, and a beam that punishes two linked players for standing too far apart. Move away from a feared ally, and run toward your link partner.

Who does what

Tanks

One tank on the Guardians near Sara in phase 1; largely unneeded once phase 3 begins.

Healers

Keep a small, mobile healer with the brain-portal team; everyone else heals the tentacle fight outside.

Damage dealers

Send your best burst players through the brain portals to clear each vision quickly and hit the exposed brain hard.

5.The Celestial Planetarium

Boss 14

Algalon the Observer

The Celestial Planetarium

2 tanksTimed, weekly accessEnrage at 6 minutes

A secret, timed fight: reaching him requires the four Keeper hard-mode quests done first, and he enrages and wipes the raid six minutes after the pull, so every phase runs on a tight schedule. There is no separate "hard mode" toggle here — the whole fight already is one.

The phases

Phase 1: 100% to 20%Stars and Constellations

Manage Collapsing Stars and Cosmic Smash while a single player kites the Living Constellations into Black Holes to remove them.

Phase 2: 20% to 0%Dark Matter

Stars and Constellations stop spawning. New Black Holes appear in a fixed square, each releasing an add the off-tank must hold while the raid finishes the boss.

Mechanics

TankQuantum Strike and Phase Punch

Heavy, frequent tank damage plus a stacking debuff that phases the tank out for ten seconds at five stacks. Swap at three stacks so nobody gets phased out unexpectedly.

AddCollapsing Stars

They lose health on their own and explode in raid-wide shadow damage if left alone. A dedicated ranged DPS kills them one at a time so the raid can prepare for each explosion, and each kill leaves behind a Black Hole.

MoveCosmic Smash

Marks up to three players with a fissure that explodes for damage based on distance. The whole raid, not just the marked players, moves away from the impact point together.

DeadlyBig Bang

Every 90 seconds, a raid-wide lethal cast. Everyone shelters inside a Black Hole except one designated soaker (a Shadow Priest using a damage-reduction cooldown works well), who eats it in the open so the boss cannot reset from an empty room.

Who does what

Tanks

Two tanks trading Phase Punch stacks; the off-tank also picks up Unleashed Dark Matter adds in phase 2.

Healers

Keep the tank topped between swaps, and be ready for the raid to reset threat every time someone returns from a Black Hole.

Damage dealers

Track the six-minute enrage timer and pace cooldowns so the last big push lands before it.

The enrage is final

Ascend to the Heavens wipes the raid outright about six minutes after the pull, or immediately if everyone phases out of the normal realm at once during Big Bang. Always leave one soaker behind.

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