S1·E64 Golden Disks of Knowledge

The Sorceress summons He-Man to Castle Grayskull, which appears to be being haunted, the jaw bridge smashing repeatedly against the edge of the abyss, and the eyes flashing! Inside it is even weirder, full of grotesque holographic images of demons! The Sorceress has no idea what is going on, and Orko suggests that someone from the spirit dimension may be trying to contact The Sorceress. Using the window of the ancients, The Sorceress manages to trace the source of the haunting- the spirit of a magician called Zanthor, who was formerly a member of the Council of the Wise, but was exiled into the spirit world when he sold the Golden Disks of Knowledge to Skeletor, giving him the knowledge he needed to enter Eternia and build Snake Mountain. Zanthor wishes to amend for his crime by helping He-Man get the Disks back.

So The Sorceress summons Zodac- the only remaining member of the Council of the Wise- and Zodac grants Zanthor temporary entrance into Eternia, though he must remain in spirit form until he can prove he is sincere. Zanthor then accompanies He-Man, Battle Cat and Orko to Snake Mountain, to find where Skeletor is keeping the Golden Disks of Knowledge.