The Immerseus encounter has two phases that alternate until the boss
Immerseus has a Corruption resource bar, which starts out at 100 at the
beginning of the fight, and which then decreases progressively as the fight
goes on, based on actions performed by your raid. Immerseus dies when his
Corruption level reaches 0.
The Immerseus Phase is the phase in which the fight starts and it
consists in fighting against Immerseus himself, who is stationary in the
center of a circular arena. When Immerseus reaches 0% health (something that
will happen quite quickly, since he has a small health pool), he will split
into many adds, and this will trigger the second phase.
The Split Phase starts when Immerseus reaches 0% health. The boss
splits into many small adds (some friendly, some hostile) which then slowly move
towards the center of the arena. The friendly adds can be healed, and the
hostile ones can be damaged and killed. When all the adds have been accounted
for (having been healed, killed, or having reached Immerseus), the Split
Phase will end, and the Immerseus Phase will re-start.
Each add killed or healed during the Split Phase removes 1 point of
Corruption from the boss, and when the Immerseus Phase restarts, the health
percentage of the boss will be equal to his Corruption level. The implications
of this are twofold:
You need to bring the boss to 0 Corruption to kill him.
The Immerseus Phase will be shorter and shorter as the fight goes on.
INFORMATION IS TAKEN FROM: http://www.icy-veins.com/immerseus-detailed-strategy-wow