Boss 1 of Siege of Orgrimmar opens with Immerseus, a fairly straightforward boss fight focusing on mechanics.

Immerseus is a TWO(2) phase fight consisting of two forms: Tears of the Vale, and Split Form. The goal is the "cleanse" Immerseus' corruption and not allowing said corruption to rejoin his physical form.

Phase 1: Tears of the Vale

Immerseus starts in this phase. In this phase he has 3 abilities that need to be taken into account.

1. Corrosive Blast: A frontal cone blast that hits for 500k+. Also leaves a debuff that increases damage taken from Corrosive Blast by 100%. Tanks will have to swap each blast to avoid any insta-gibs.
2. Sha Bolts: 125k Shadow Bolts that leave behind pools that tick 65k per second. Positioning of bolts around the room will be crucial. Pools despawn when Immerseus reaches Phase 2.
3. Swirl: Similar to Wise Mari or Hoptallus, this is a rotating damage beam that causes a knockback. The beam is a close blue color to the rest of the room as well as Immerseus. Identifying which direction will be crucial.

When Immerseus is DPS'd down to 0, Phase 2 will begin
Phase 2: Split
Immerseus starts to split when taken to 0 in Phase 1

1. Upon reaching Phase 2. Immerseus will split into many blobs. There are two types of blobs: Sha Puddles and Contaminated Puddles
2. Sha Puddles: Similar to Heroic Protectors, upon dying applies a "debuff" in a 10 yard radius increasing damage dealt by 25% to OTHER Sha Puddles.
3. Contaminated Puddles: Reversely, these puddles need to be HEALED up to 100% to despawn. As their health increases, their speed decreases. When healed to full health, all players with 12 yards will receive 25% mana. THIS IS ESSENTIAL.

If either type of puddle (blob) reaches Immerseus, they will deal 75k damage to the entire raid.

Conversely, each blob that does not rejoin with Immerseus will remove 1 Corruption from his existing Pool of 100. The goal of the fight is to completely cleanse the corruption meter.
After all blobs have either died or reached Immerseus, Phase 1 will restart until the fight is complete.