Somnus

Somnus is a level 62 elite green drake found in the Swamp of Sorrows. He is a champion drake of the green dragonflight. Presumably, Somnus is associated somehow with the Green Flight's purpose within the Temple of Atal'Hakkar.

He is one of the four drakes that must be killed for the Horde side of the Onyxia-key quest chain. Emberstrife wants Somnus' skull.

Apparently, he causes problems to lower level players who are trying to quest in the area.

Trivia
Somnus is a Latin noun for sleep. Wiktionary

Objective of

 * Drops The Skull of Somnus for the quest.