Vashnik is the third or fifth boss of The Venomous Abyss raid, depending on your route.
It is a one-phase fight that repeats mechanics until defeated. There are three different Fountains around the arena: Blood, Shadow, and Flame. Vashnik occasionally casts Imbibe, drawing power from two nearby fountains, creating a venom, and gaining Infusion stacks for that last 1.5 minutes. Infusion stacks increase the damage of mechanics associated with the respective venoms, making it important not to empower Vashnik with too many Infusion stacks of a specific venom.
Note the three snake fountains around the edge of the room (blood, fire and shadow). The boss needs to be positioned between two of them.
Once the boss reaches 100 energy, the boss will move to his next location between two other fountains. Fountains selected twice in a row get empowered.
Each fountain spawns adds that try to get to the pool in the middle (kill them) and applies a debuff.
Red Blood: add CAN'T be CC'd and splits into smaller adds when killed. Debuff is a heal absorb that can only be healed off by having several other players stand in your circle.
Purple shadow: bunch of adds that CAN be CC'd and spawn puddles on the ground when they die. Debuff causes players to drop exploding puddles.
Orange Fire spawns two adds that CAN be CC'd and pulse damage. When they die they cause massive damage and dot to the raid (stagger kills?). Debuff - large orange pulsing area around one player - move away from raid.