Gormungandr, Serpent of Arreat is a unique Demonic Serpent sometimes found in Arreat Crater Level 1 in Act III of Diablo III.
It has the Frozen Pulse, Frozen and Plagued traits.
A bounty may be offered to kill Gormungandr, in which case the secondary objective is to kill 50 enemies.
Trivia
The name of the creature is a reference to the World Serpent Jörmungandr in Norse mythology.