Fathom Dweller

EDIT 7th Sep: Read post by Blackfaded or Eipril. The boss is summoned using a particular method of activating orbs in the correct order, and the mount and pet rewards seem to be based on a weekly quest.

Information:

Kosumoth the Hungering is located in the center of the physical Eye of Azshara island in the south of the Broken Isles (NOT the dungeon version).

Kosumoth has 83.42M health baseline and requires a group to defeat. His health increases/scales up with more than 5 people. He has 3 main abilities; Shadow Bolt Volley, Gluttonous Shadows and Void Fissures as well as melee attacking:

1. 
   
2. Shadow Bolt Volley is the most deadly. It is a 2 second cast that that deals around 10M damage to all enemies (eg: wipe damage) so NEEDS to be interrupted. It seems to be cast every 8 seconds so a rotation of two interrupters should be fine.
3. 
   
4. Gluttonous Shadows is a 15 yard radius AoE around Kosumoth that deals a large amount of shadow damage to anyone stood in the area and may also heal him(?). This damage isn't deadly, however the cast can be interrupted so if you have spare interrupters they should interrupt this. The spell has a 2.5 second cast and seems to be cast randomly every 15-25 seconds.
5. 
   
6. Void Fissures is a 1.5 second cast that summons 8 small adds. These adds have low health and don't really seem do much except melee-attack, so should simply be quickly AoE'd down. This ability seems to be cast every 15 seconds or so.
7. 
   
8. Apart from his main 3 abilities he also melees at a slow and unsteady pace. This damage however isn't significant and any healer should have no issue keeping the tank alive.
9. 
   

Here is my YouTube video, which has two kills where you can see Kosumoth and his abilities in action: https://www.youtube.com/watch?v=isDeBtus-Kk

Looks like it increases swim speed by 100%. Atleast that's where I'd put my money at.

What a unique mount! Looks like you might get dizzy riding it, though.