I posted some time ago during the 3.3 PTR about the proc on the ICC10 tanking trinket. I concluded that the proc was rubbish, and to avoid using the trinket. As it happens the trinket was changed significantly before launch (perhaps GC is a reader! /wave!).
The latest version is the Unidentifiable Organ, which drops from Professor Putricide, and it's really quite interesting. Leaving the armour aside for a moment, let's look at the equip effect.
"Each time you are struck by a melee attack, you have a 60% chance to gain 24 stamina for the next 10 sec, stacking up to 10 times."
This means that the probability of this proccing in any 10 second period is calculated as follows:
Let S = Boss swing speed, A = Tank combined dodge/miss/parry, E = Expected number of tank hits in a 10 second period, P = Proc chance in a 10 second period.
E = (1 - A) x 10/S
P = 1 - 0.4^E
Plugging in some figures, let's have combined avoidance of 35% including Icewell Chilliance, and a boss swing speed of 2.5 seconds. This gives the expected proc chance of 90.76% every 10 seconds.
So, iterating this, the chance of it stacking to 2 is 90.76% x 90.76% = 82.39%. The chance of a 3-stack is 74.78% etc. Based on some very rough and ready stats, I would say that with those figures your average stamina bonus would be aound 7 stacks - so 168 stamina (the probability of a 7-stack is around 50% - it's been a while since I've done any stats and so it might not follow that the probability of 50% translates to the expected value so that figure of 7 could be wrong). You will get a different result with different bosses and different levels of gear however.
The combination of armour and stamina makes this one of the most powerful EH trinkets around - it is arguably better than either of the 25 man trinkets (this one is an avoidance trinket, and the regular armour proc is great but armour is less powerful at low health than at high health so it's more of an overall mitigation thing than something which is likely to save your life; and this one is great but very situational). The usual caveats apply however:
- the stack will probably fall off at some point during the fight, so you might be unlucky with the timing and it could happen during a period of high boss damage
- for tank swap fights such as, well, most of them, the stack will fall off when not tanking
- if the boss stops attacking to cast a spell, the stack is more likely to fall off
- caster bosses in general are less likely to trigger it since spells don't give the trinket a chance to proc
It's worth noting that this trinket will be especially useful for tanking Saurfang during his Frenzy, when his attack speed increases sub-25%.
Getting over idleness
1 day ago