As the first boss of the raid, it should come as no surprise that Immerseus is very straightforward, but the mechanics, though simple, deal heavy damage if not dealt with correctly. Player are essentially trying to cleanse Immerseus' corruption by beating the corruption out of him, and killing it before it can get back. He has two phases, Tears of the Vale and Split. Immerseus will begin the fight in Tears of the Vale, and the object is to DPS Immerseus down to zero health, when he will shift into his Split mode.
Tears of the Vale
This is the main phase, which Immerseus starts off in, and spends the majority of the fight in. There are three abilities that players need to know about, all of which are simply handled with movement. Firstly, the tanks will be repeatedly assaulted with Corrosive Blast. It's likely best for tanks to swap after each blast or two to avoid being one-shotted by this ability, and it is a very, very bad idea for anyone who isn't a tank to stand behind the tanks. It's a cone effect, and will hit the area around the tanks as well, this is not a targeted ability.
As well as the corrosive blast, Immerseus will periodically target every player with Sha Bolts. These target the player, and leave a small pool on the floor. These pools will only disappear once Immerseus enters his second phase, so it is important for players to position these pools in such a way that they don't paint themselves into a corner with the damaging pools. On the PTR at least, they hit very hard. In 10-man, this shouldn't be too much of a problem, but in 25-man it could be a bit tricker. It will likely be the case that there will have to be a ranged area of pools and a melee area of pools in each segment of the room that isn't occupied by the tanks. Speaking of the tanks, it is very easy for healers to accidentally get themselves out of range of the tanks, thanks to the pools and the cone effect. Consider positioning carefully, and perhaps use raid markers and the like to easily handle the positions of the raid.
The final ability which Immerseus uses is Swirl. It's a simple ability, a beam effect that he channels out of his front-facing side, and while it's channeling, he slowly rotates around the room. However, the beam is blue. The room is blue. The floor is blue. I'm sure you see where I'm going with this! The beam is extremely hard to see. I didn't really get any good screenshots of it, but the best I've been able to find is below:
Immerseus has both a health and a corruption bar. His health bar is easily emptied, and doing so is what causes the phase change, but what you're actually trying to do is empty Immerseus' corruption bar. Once his health reaches zero, the corruption will fly out of him in the form of little colored blobs. Rather like T-1000 in Terminator 2, Immerseus' blobs will attempt to re-form, and it is the player's responsibility to do their best to ensure that that doesn't happen. There are two types of blob, Sha Puddles and Contaminated Puddles. The Sha puddles need to be DPS'd down, and when each one pops, it will cause Sha Residue. You'll probably notice that this increases the damage dealt to other Puddles by 25%, so it's worth bursting one down to get the buff up. These also deal damage to nearby enemies on explosion.
If the Sha Puddle blobs reach Immerseus, they will deal 75,000 shadow damage to the raid, and the same applies for Corrupted Puddle blobs. Purified Corrupted Puddle blobs, that is to say, Corrupted Puddles which have been healed to full, will deal only 30,000 frost damage. As a side note, these blobs aren't puddles, they're blobs. Calling them puddles in a fight where there actually are puddles is daft.
Anyhow, and Sha Puddle (blob) which is killed, and any Corrupted Puddle (blob) healed to full will reduce Immerseus' corruption bar by one. He has 100 corruption at the start of the fight, and cannot gain any corruption back, so all you need to do is kill or heal 100 blobs. Or 100 puddles, to use the proper term. Once the phase is over, Immerseus goes right back to Tears of the Vale. It's a simple, fun fight, my only complaints are the visibility of the Swirl, and the names of those adds!
Do also check out our previews for Kor'kron Dark Shaman, and General Nazgrim.