The Spirit of Grayskull
first appears in the first wave of Masters of the Universe Minicomics
as a skull-shaped cloud of smoke that emerges from Castle Grayskull
and speaks to the Heroic Warriors
as thye voice of the castle itself.
The Spirit of Grayskull also appears in a Filmation episode as a disembodied head who talks with The Sorceress of Castle Grayskull.
A translucent King Grayskull figure was sold as the Spirit of Grayskull for Masters of the Universe Classics, thus defining the Spirit of Grayskull as King Grayskull's spirit.