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. 

• 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/leach circle 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. (if empowered, adds have more health)  

    • Debuff is a heal absorb and swirlies to dodge.

  • Orange Fire spawns two adds that CAN be CC'd and pulse damage.  When they die they cause massive damage (Heroic: causes a stacking debuff).  

    • Debuff - large orange pulsing area around one player - move away from raid.

  • Pull boss towards one of the sets of adds and cc or ranged dps the others.

  • Plague Froth: random players will get four green lines from them .. move out.  Lines produce blob waves.

  • Tank: Dripping Fangs: physical damage, plus nature damage dot, taunt swap.

  • Kill all adds and, before the boss gets to full energy, move him to next position.

Visual aid: https://raidplan.io/plan/362cskggepxz8fwx