Old Serra'kis

Old Serra'kis is a level 25 huge threshadon hidden mini-boss found in the Forgotten Pool of Blackfathom Deeps.

Dungeon Journal
Many of the creatures swimming beneath the Moonshrine Ruins were killed for food after the Twilight's Hammer cult took up residence, but the largest thresher in the water proved to be uncatchable, and those who persisted in hunting it inevitably died in the attempt.

It appears that Old Serra'kis was driven out by the Guardian of the Deep so it can spawn eggs on its territory.

Strategy
Finding Serra'kis is harder than the actual fight. To reach him, you must dive into the water around Moonshrine Sanctum and swim under the left side of the ruins. Thankfully the fight isn't tough and as long as you're careful not to run out of breath you shouldn't have any problems. Having a Warlock or Shaman to cast Water Breathing is helpful here. If you are unable to do this, it will be difficult to kill him quickly enough because the cave he is located in does not have an opening to the surface for air. Kiting him out of the cave may be a better option than fighting him inside it in this case.

Also note that Serra'kis regularly heals himself for a small portion of his health using a melee ability. With decent dps, this will not be a problem. Those groups with unusually low dps may not be able to overcome his healing, as can be the case when a weak group of 3 or 4 attempt Serra'kis.

Abilities

 * Serra'kis leeches health from all nearby players, inflicting Shadow damage and healing himself every 4 seconds.
 * Serra'kis leeches health from all nearby players, inflicting Shadow damage and healing himself every 4 seconds.