Vasella is immune to damage and slow, she is also tethered to the room.
Vasella will summon a random player to her every 1 minute or so. That player has 15 seconds to get out of line of sight of the boss, otherwise everyone takes 50k dmg.
Upon engaging 2 adds will spawn that have very high AC, but low health. They are vampires that drop at 50% and need to be staked. They have a special ability that dispels their target of all buffs/discs. In addition, if these adds get too far from Vasella, they will cause an emote and aoe damage. Further, if they die within LoS of the boss, then Vasella will power up.
Tank her inside the circular room, and get her right behind a pillar next to the door. This way she is very near the door but cannot see the door. The tank stays in this room alone with the boss. The raid, meanwhile, stays right outside the door. An offtank or two, will hold the adds right there in the middle of the raid. The adds can be affected by nearly every spell in the game. Mez, stun, slow, charm, etc. Use crowd control methods and kill them one at a time. Vasella takes damage every time an add dies or a toon gets out of her sight after she summons them.
