You see a nightmare of Gaz'haragoth
Notes
Looks the same as a Terrorsleep. You will find them by stepping in a Strange Vortex, created when Gaz'haragoth's minions are slain.
Abilities
Damage Taken From Elements
Physical 
Death
Holy
Ice
Fire
Energy
Earth
Location
Behaviour
Unknown.
Strategy
Ignore them and run to the portal in the south-east. These creatures have instant respawn, after you killed them, they spawn again in a moment, so they should be avoided at any cost.