Callisto is an enormous bear that resides south of the Demonic Ruins, and is the only monster to drop the tyrannical ring. He attacks using melee. He can knock players back a few tiles, which renders them unable to move for a few seconds, as well as heal himself slightly. He is also able to perform a deadly shockwave attack that can hit up to 60.
Callisto is immune to Magic damage, although spells with effects such as Vulnerability will still affect him.
Callisto is named after one of the Greek god Zeus' many lovers, who was turned into the constellation Ursa Major.
Upon release, Callisto had a slightly worse drop table. In addition, players could exploit his low Magic by safespotting him with the trident of the seas behind a cluster of skeletons north of him. The cluster of skeletons have been removed and Callisto is now immune to Magic damage.
He also used to be able to use his knock-back attack more often. This was changed in an update on 8 February 2018.