You must dive into the water around Moon-shrine Sanctum in game

Old Serra'kis is a giant threshadon found in Black-fathom Deeps. Finding Serra'kis proves to be harder than the actual fight. To reach him, you must dive Aion kinah into the water around Moon-shrine Sanctum and swim under the left side of the ruins.

The fight is not tough, and as long as you're careful not to run out of breath you shouldn't have any problems. Having a Warlock to cast Unending Breath or Shaman for Water Breathing is helpful here.

Without either of these classes, it will be difficult to kill him quickly enough due to his locations not having an opening to the surface for air. Coaxing him out of the cave may be a better option than fighting him inside it in this case.

Aku'mai is a giant hydra and acts as final boss of Black-fathom Deeps. According to legend, Aku'mai has taken up residence within Black-fathom Deeps, which once was a glorious temple dedicated to the night elves' moon-goddess Elune. Aku'mai, a favored pet of the primordial Old Gods, has preyed upon the area ever since.

Drawn to the hydra's presence, the cult known as Twilight's Hammer has also come to bask in the Old Gods' evil presence and sacrifice innocents to her to gain power. Aku'mai is not an Old God, just one of their favorite pets.