Bloodlust is one of the key abilities that when properly used can greatly ease an encounter. But, since it's only available once per fight (LK Heroic excepted), it needs to be timed properly. There are different views on how to time it. The naive claim is that it should be popped when the boss is below 35% HP, because several classes deal more damage then (the so-called Execute-phase). However, this turns out to be false.
Execute-phase or not
When you Bloodlust in the Execute-phase, the time you spend in this phase goes down, which means that the raid has less time to benefit from that sub-35% DPS boost. It turns out that this effect exactly compensates for the increased damage you get from Bloodlusting during the sub-35% phase. And since math says more than a thousand words, I'll show this.
The following variables are used:
- H = boss HP
- R = base raid DPS without Bloodlust or sub-35% bonus
- D = duration of Bloodlust (40 sec)
- e = raid DPS increase factor due to Execute-phase (1.2 means 20% increase)
- b = raid DPS increase factor due to Bloodlust (1.2 means 20% increase)
- t1 = time from start to the moment the boss hits 35% HP
- t2 = time from the moment the boss hits 35% HP to the moment of death
- t3 = time to kill the boss, so t1 + t2
If we Bloodlust early (say, at the start of the fight), then we have some time with Bloodlust, during which we deal D * R * b damage. So the time left to burn the boss to 35% is (0.65 H - D * R * b) / R. And we need to add D to this to account for the time we spent during Bloodlust. Then we can obtain the time needed to burn the last 35%: (0.35 H) / (e R). Adding this up gives the total time:
If we Bloodlust when we hit 35%, the calculations follow the same idea:
Notice anything special? Right, both results are the same. The conclusion is that whether you Bloodlust early or late, the time-to-kill remains the same.
So when do I use Bloodlust?
Seeing as there is no DPS-gain from waiting for 35% to Bloodlust, I can give some compelling arguments to Bloodlust as early as possible, once all the relevant debuffs are up and people have their rotation going. This way players can pop their trinkets and cooldowns early and automatically have them sync up with Bloodlust, which is a decent DPS gain. If you postpone Bloodlust until the 35% phase, people will either not have their cooldowns available or they will have postponed using those cooldowns, which makes them get less benefit from them over the entire fight.
Another reason to Bloodlust early is that people die from time to time. Especially on new bosses, where DPS actually matters, you may lose people during the fight. At the start, everyone is alive and at full capacity, so you get the maximum out of your Bloodlust.
All the conclusions made earlier assume a simple tank-and-spank bossfight with no adds or different phases. This is not entirely realistic and in actual fights, there may be circumstances that make you want to adjust your Bloodlust timing.
Encounter-specific damage buffs (such as Icehowls stun) are a great thing to combine with Bloodlust. A dangerous wave of adds is more easily disposed with a Bloodlusted raid. And finally, don't forget the healers: Bloodlust makes their work easier too, so if you anticipate a period of very high raid damage, Bloodlust can help the healers get through it.
ICC Boss-Bloodlust Guide
A quick summary of when to Bloodlust (IMO) on ICC fights:
- Lord Marrowgar: Preferably shortly after the pull or after the first Bonestorm.
- Lady Deathwhisper: Start of phase 2. Alternatively, during phase 1 after clearing up an add-wave.
- Gunship Battle: People Bloodlust here?
- Deathbringer Saurfang: At his 30% Frenzy. This phase is the most dangerous, so you want to get it over with quickly.
- Rotface: Just after the pull, people will be busy running around to drop their oozes later on.
- Festergut: Just after the first tank-switch. The old tank will be DPSing with alot of damamge-increasing buffs and Festergut will hit 3 stacks of Inhale Blight, causing alot of damage to be healed.
- Professor Putricide: At the start of phase 3 (normal mode) or during the second double-ooze transition phase (heroic mode)
- Blood Prince Council: Once Taldaram or Keleseth gets empowered. Empowered Valanar requires too much movement.
- Blood Queen Lana'thel: At the end of the fight, just after the last round of bites and after the second fear/airphase. More vampires means more DPS.
- Valithria Dreamwalker: Your healers decide, but generally once they have high stacks of their buff and just after they exit a portal.
- Sindragosa: Phase 1. We like to stress that Phase 3 is not a DPS race and just maximize the time we have for that phase by getting through the first 65% of her health quickly.
- Lich King: Second transition phase (normal) or right at the start and again in Phase 3 (heroic).