He was one of the four drakes that had to be killed for the Horde side of the Onyxia-key quest chain. Emberstrife wanted Somnus' skull.
Apparently, he caused problems to lower-level players who were trying to quest in the area.
Abilities[]
Corrosive Acid Breath — Shoots a cloud of acidic breath at enemies in a cone in front of the caster, reducing their armor and inflicting Nature damage every 5 sec. for 30 sec.
Sleep — Puts an enemy to sleep for up to 10 sec. Any damage caused will awaken the target. Only one target can be asleep at a time.
Strike — Strikes at an enemy, inflicting increased melee damage.
Wing Flap — Strikes enemies in a cone in front of the caster, knocking them back.
Somnus
Corrosive Acid Breath — Shoots a cloud of acidic breath at enemies in a cone in front of the caster, reducing their armor and inflicting Nature damage every 5 sec. for 30 sec.
[The Skull of Somnus] for the quest 
[60] The Test of Skulls, Somnus.
[43] Pool of Tears