So there seems to be zero guides or strategies on how to defeat the last boss in The Dread Vaults so I thought I would make one. The fight is REALLY simple once you understand the core mechanic, something that took my group over an hour and a half to work out.
You basically kill all the adds that are summoned by Yshiggol?AWAY?from himself. They have a purple glowing link connected to each add so you know when it is too close to the boss. If it is killed near the boss he will heal for the amount of health of the mob, which is a large chunk.
At 70%~ he will summon a miniboss which can be CCed and must be killed along with dpsing the adds that normally come down. All he does is simply teleport to a random party member and perform a slow swing that does a lot of damage and is easily avoidable. Back to Phase 1 until he hits 30%.
At 30%~ he will summon another miniboss, a spider, which might be CCable (need clarification) that also summons adds throughout this phase that need to be AOE?d down. She does a long range frontal cone spell which can also be easily avoided as well as medium circles on the ground that explode after 2-3 seconds. Proceed to dps down until its dead. Repeat Phase 1 until boss is dead.
Ignore tentacles the entire fight as they will not do anything to you and will respawn after a few minutes.
Hope this helps anyone who was having issues with this and saves you some time.