Chillhoof is the final boss of the Icedeep Caverns. The Icedeep Caverns are a two-floor dungeon in Act I of Torchlight II, located in the Frosted Hills.
Once killed, he drops the Master Key, which is needed to open his Forgotten Chest.
Killing him earns players the achievement The Big Chillhoof.
https://torchlight.fandom.com/wiki/Chillhoof