The Ancient War Bat is a Boss that can be found in The Frozen Tunnels. It is known for being the only place to obtain the chilly Glacial Helm, Glacial Body, and Glacial Legs.
The Ancient War Bat deploys spikes under your character which after 1.5 second(s) grows and traps anything in it, stopping it in its tracks. (This can cause up to 1300 damage)
This can be obtained as an extra item roll rarely from all monsters. They will only drop if the monster is near your level (bosses not included)