Immerseus is the first encounter in the Siege of Orgrimmar raid instance, and First encounter in the Vale of Eternal Sorrows LFR.

For a detailed view of his abilities and loot table across LFR/Flex/10/25 please CLICK HERE

General Strategy/Overview

Immerseus begins the fight with 100 Corruption, which is noted as a power/energy/rage style bar.  The objective of the encounter is to cleanse him of his corruption all the way down to 0.  Each time that Immerseus is reduced to 0 health, he will split into a mixture of dark Sha Puddles and watery Contaminated Puddles.  The puddles will begin moving back to the center of the room.  The DPS must destroy the Sha Puddles, and healers must heal the Contaminated Puddles to be able to purify them before they reach the center.  Following each "Split", Immerseus will reform with reduced health and Corruption based on how well the raid performed during this period.  This cycle will rinse and repeat throughout the fight until he is cleansed.

Phase 1 - Water Elemental

Primarily, tanks need to tank him so he does not directly face any raid member.  His "Corrosive Blast" is a conal ability that deals high damage and gives a stacking debuff (lasting 45 secs) that increases shadow damage taken by 10% per stack.  Requires trade tanking.

He will often cast "Sha Bolt", dealing low-moderate shadow damage to all raid members and leaves a small dark pool on the ground, move out of it to prevent extra damage, i.e. stay out of "fire". Only move as far as you have to, cause the pools to overlap to give yourself and the raid more freedom to move for his next ability.

His next ability is "Swirl" this will spawn several lighter blue pools and move along the floor fairly quickly, ensure you give yourself enough room from the "Sha bolt" pools to be able to dodge them safely.  This will deal low shadow damage but knock you back.  Could be detrimental if healers constantly get hit by this.

For Melee players, do not get too close to the boss or you will get hit with "Seeping Sha", this is noted by the large pool surrounding him.  This will hit for moderate dmg and knock you back.

Phase 2 - SPLIT

Again, once he reaches 0 health, he "Splits".  Any dark or watery pools that are still up from the "Sha Bolts" or the "Swirl" will immediately go to the middle of the room.  LET THEM, these do nothing. 

The primary pools you need to watch are the Sha Puddles and Contaminated Puddles.  The Sha will be dark, the Contaminated will be more watery in color.  For every Sha Puddle killed, and Contaminated Puddle cleansed, he will lose 1 percent of his remaining Corruption bar.  When he re-forms, his health should be equal to the amount of corruption he has.  So if he is at 77 stacks of corruption after a re-form, he will start at 77% health, etc.

Sha Puddles - he will form 1 Sha Puddle for every 4 units of corruption he has.  So on the first split, since he has 100 corruption, he should spawn 25 Sha Puddles.  That is based on Dungeon Journal entry, but from videos it doesnt "appear" to be that many, it also contradicts the next part of the journal entry as well by watching the video.

Once a Sha Puddle is destroyed, it will give "Sha Residue" to players within 10 yards, dealing 25% increased damage to other Sha Puddles.  This STACKS!

Contaminated Puddle - he will form 1 Contaminated Puddle for every 4 units of corruption that have been previously removed.  This is where the dungeon journal and video are contradictory.  Based on the journal entry, on the first Split, there should be NO Contaminated Puddles to cleanse  since he is at full corruption, but the video clearly shows Contaminated Puddles available to be cleansed.  It may not be like that "live" and they may have only had them available for PTR testing purposes.  Regardless, once these are healed/"cleansed" they will turn a dark blue color.

When a Contaminated Puddle is healed, it will progressively slow down in proportion to the amount of healing it has received until it is fully "cleansed".  Once a Contaminated Puddle is cleansed, it will restore 25% of mana to allies within 10 yards and increase healing done by 75%, this STACKS!

Once either of the puddles reach the center of the room, Immerseus will pulse AoE dmg to the raid, based on the type of puddle that reaches him.  If a Sha OR Contaminated Pool reaches him, he will pulse for around 100-110k shadow damage.  Once a Purified Pool (yes, one that is fully healed) reaches the middle, it will only pulse for around 40-45k.  So ideally you want to kill all of the Sha Puddles and heal as many of the Contaminated Pools as possible, so when they any remaining get re-absorbed you dont wipe to the AoE damage.

TLDR (go back and read it)

This fight is fairly straight-foreward and shouldnt take long to get past.  Tank swap when appropriate, assuming 2 stacks.  Stay relatively spread, and overlap your own Sha Bolt pools so you have room to move.  When he splits, DPS the dark pools and HEAL the watery pools so you have less AoE to deal with on the re-absorb/re-form.  Raidwide defensive CDs are likely best used later in the fight. There will likely be more Sha Puddles up dealing more damage in the beginning, but at the end, it will be harder on the healers as there will be a LOT more Contaminated Pools to heal, more than the healers can handle by themselves, so there MUST be hybrid healing on them at the end. Overall it will best be determined how your raid make-up is and where you feel it will benefit the most.