Yogg-Saron is the last regular boss in the Ulduar instance.  He is an exceptionally complicated boss to defeat with three separate phases each requiring coordination, movement and skill from all players involved. This is the boss that many players have issues with and the reason many guilds still have not cleared Ulduar. If you are still having difficulty check out our guide for all the details.

Crusher Tentacle – (Phase two only) These tentacles continually attempt to channel Diminish Power which reduces all damage caused by roughly 20% to anyone in the room, this ability stacks from up to 4 tentacles.  It can be interrupted by being hit by melee.  Each time they are hit by melee though they gain a focused anger buff which stacks to 100.  Each stack increases damage dealt and attack speed by 3%. If melee is in range while angered, the tentacles will attack them instead of channelling Diminish Power.

You can find all the details here: Yogg-Saron Strategy Guide.