Temple of Sethraliss is a dungeon whose entrance is located in Vol'dun in the World of Warcraft game. The level range for the normal dungeon is 110-120. The dungeon contains 4 bosses. The end boss of Temple of Sethraliss is Avatar of Sethralis.
Merketha is a giant snake that only appears once the whole room has been cleared of trash. There are two main phases of battle and then swap between the two. Phase one will see you fight Merketha on his own. Sometimes he will produce a flash to disorientate you, other times, it'll just be a simple case of nuke the boss down.
The other phase is that Merketha will burrow into the ground and attack you but sadly you will not be able to attack back. You will need to deal with Merketha's offspring, smaller versions of itself, once the adds have been killed, Merktha will reappear.
When you enter the final boss zone, the boss will be held in a prison like object above an object. A number of humanoid serpents will be holding the Avatar in the air, you need to kill the serpents before the fight can start.
The fight is a near copy of what happened before you started the fight, humanoid serpents will keep the Avatar in the air. You need to kill the serpents so that the Avatar's health can regain. In addition to having to kill the serpents, frogs will spawn which will attack you and therefore must be killed. Once the health is full, the Avatar will help you get back out of the place by sending you to the entrance if you need it.