A hydra at the bottom of Fomor.
Per patch 1.100 (12.15.09):
* Myrddraxis: This monster has been given new tricks and abilities and its Lesser Heads have been reduced in level.
Per the 1.101 Release Notes 02.24.10:
Myrddraxis: This monster is now greatly reduced in level but has been given extra hit points, defenses and a few more tricks and abilities. New mechanics have been put into place to target inattentive players.