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Hard mode, not to be confused with heroic mode, refers to the increase in difficulty of a raid boss encounter resulting in increased rewards and/or achievements. This is accomplished by doing or not doing certain things that increase the difficulty of the boss fight. You can not set the boss encounter to Hard mode from the user interface.
Prior to the release of Wrath of the Lich King and the coming of real hard mode encounters, the Temple of Ahn'Qiraj was the first raid instance to have an encounter where the difficulty and loot were determined by the order you kill each boss.
Zul'Aman was the second raid where loot varied depending on what the players did. In this case the loot was on a timer, the more bosses that were defeated before the time ran out the better loot was rewarded. In total four chests, one at each of the four first bosses were possible to obtain. The Amani War Bear from the last chest was removed in Patch 3.0.2.
Obsidian Sanctum was the first Wrath of the Lich King raid instance released with a hard mode boss encounter.
Ulduar took the concept further with 10 of the 14 bosses having hard modes. These are: Flame Leviathan, XT-002 Deconstructor, Assembly of Iron, Freya, Hodir, Thorim, Mimiron, General Vezax, Yogg-Saron, and Algalon the Observer. Algalon is WoW's first hard mode only encounter, and for this reason was made optional (skippable) as far as completing the instance is concerned.
Hard mode encounters
Three Bugs in the Temple of Ahn'Qiraj offers players 3 different methods. The easiest way to complete this encounter is to kill Lord Kri first, then Princess Yauj and finally Vem. The 'medium' way is to kill Lord Kri in second. The hardest way is to kill Lord Kri last. The rewards are determined by the order you kill the Bug Family. Each bug has some unique items in its loot table, while others are shared.
Sartharion in Obsidian Sanctum has 3 different hard modes. The three drakes, Vesperon, Shadron, and Tenebron can be killed to make the Sartharion encounter of normal difficulty or it can be increased to hard mode by leaving one or more of the drakes alive when starting the Sartharion encounter. Defeating Sartharion with the drakes up rewards an achievement and additional loot, increasing with additional drakes: / (one drake left), / (two drakes left) and / (all drakes left).
Several achievements are also given out depending on the number of towers destroyed. They are / (three towers destroyed), / (two towers destroyed), / (one tower destroyed), and / (all towers left up).
XT-002 Deconstructor has one hard mode. At 75%, 50%, and 25%, XT-002 Deconstructor's heart will be exposed. Destroying the heart (which has a multi-million HP pool) activates the hard mode, which heals XT-002 Deconstructor to full HP and gives him the buff Heartbreak, which increases XT-002 Deconstructor's damage done by 15% and his HP by 50% (60% in Heroic). Defeating XT-002 Deconstructor while killing his heart awards players the achievement / and additional loot.
Assembly of Iron
There are two different hard modes for the Assembly of Iron. The normal strategy is to kill Stormcaller Brundir last ( / ). The 'medium' hard mode is to kill Runemaster Molgeim last ( / ). The hardest way to do the encounter (and the method that rewards the best loot) is to kill Steelbreaker last( / ). The rewards are increased loot and achievements.
Hodir's hard mode requires the raid to down him in under 3 minutes, preventing him from shattering the Rare Cache in the room ( / ). As of Patch 3.1.1, Hodir will not shatter the cache if he is downed mid-cast.
Reaching Thorim within 3 minutes causes Sif not to despawn and assist Thorim in the fight. Meanwhile, she cannot be targeted and she nukes the raid. Unlike the normal mode Sif does not put the debuff Touch of Dominion to reduces his HP and damage by 25% (means 25% more damage and HP than normal mode) ( / ).
Freya has 3 different hard modes. The three Ancients, Elder Brightleaf, Elder Stonebark, and Elder Ironbranch can be killed to make the encounter of normal difficulty or it can be increased to hard mode by leaving one or more of the Ancients alive when starting. Defeating Freya with the Ancients up rewards additional loot and an achievement: / (one ancient left), / (two ancients left), / (all ancients left).
Mimiron's hard mode is activated by pressing the "Big Red Button" prior to the encounter. Pressing the button puts a 10-minute enrage timer on the encounter and increases the health and damage done of all the vehicles in the encounter by 25%, while fires spawn through the area as Mimiron's workshop self-destructs. ( / )
General Vezax's hard mode is simple: Do not use the Saronite Vapors to regain mana. If nobody in your raid makes use of the Saronite Vapors, then after 8 Saronite Shards, adds will spawn, you kill the adds and then kill General (there are only 8 Saronite Shards for the whole fight that will spawn), General Vezax will drop improved loot and reward the achievement / .
There are four different hard modes for Yogg-Saron. The normal mode involves using the assistance of the four keepers - Freya, Hodir, Mimiron, and Thorim. The hard modes involve using the assistance of fewer Keepers. The reward is improved loot (assumed) and four achievements depending on which difficulty you engage him on. The achievements are: / (use three or fewer Keepers), / (use two or fewer Keepers), / (use one or fewer Keepers), and / (do not accept the help of any Keepers).
Algalon the Observer
Algalon the Observer is unique in that he has no normal mode. He is considered a "hard mode only" boss by Blizzard because to confront him you must defeat multiple bosses on hard mode.