In the Demiplane of Blood: Part of the Tier 3 Demiplane event "The Master".
In the Tower of Frozen Shadow: Part of the final encounter within the Tower of Frozen Shadow anniversary raid (see this quest entry for raid information).
In Castle Mistmoore: Makes a non-combative appearance in the "Summoning the Master" anniversary group mission.
In the Mob Graveyard: Was previously known to make rare appearances in Castle Mistmoore (perhaps only as part of GM events).