The Garngrath. Less so a monster, and more so a name picked out of a random fantasy name generator.

Good monster and definitely a worth while CR 20 monster for a group of players just venturing up to level 20, without too much min/maxing. Clocking in a decent AC, an inherent miss chance, decent saves, SR 33, Plane Shift for its self so it can be a recurring threat if you'd like, and DR 15/epic.