Luckily, ever-resourceful players and dataminers have acquired information about spells and abilities related to bosses in Ulduar from the 3.1 PTR files. There's a lot of of stuff in here -- some of it may make it into the live game, some may not, and some may be related to vehicles or other sundries involved with encounters. I'm going to go into detailed analysis of those encounters for which detailed spells exist.

It goes without saying that these will be huge spoilers to anyone who doesn't want their Ulduar experience to be anything but pure, so don't read past the jump if you don't want to know!

The datamined information comes from this thread on the official forums, with links to the appropriate spells on Thottbot.

Yogg-Saron

Dominate Mind - Mind controls a target temporarily and reduces Sanity by 20.
Low Sanity Screen Effect - Just a visual.
Psychosis - 20000 Shadow damage and reduces 20 Sanity.
Sanity - 100 stacked buff, if it reaches 0 you are permanently mind controlled. One Sanity regenerates per 3 seconds.
Touch of Dominion - Subjugates the target to the will of Yogg-Saron, weakening it (-10% HP and damage), yet "conferring a subtle control". Appears to stack every 180 sec.

Preliminary analysis:
Yogg-Saron's fight appears to be a sanity-based encounter, and if you're not already thinking of Eternal Darkness, we're not friends. There's a sanity meter, a mind control/reduced effectiveness debuff, even a low sanity visual effect. Your sanity regenerates by one point every three seconds, and it ticks down as you engage him, take damage from him, or are temporarily mind-controlled. If it reaches zero, you're permanently under the thrall of Ol' Yoggy.

Hodir

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