"The Warder of Knowledge"

by Richard F. Searight
originally published Tales of the Lovecraft Mythos, Fedogan and Bremer, 1992

Gordon Whitney is a child prodigy who grows up to study the occult.  His study of the "Eltdown Shards" leads him to a ceremony to summon "The Warder of Knowledge", a being that will lead him to enlightenment. He gives it a try.  It opens the way to a journey through vistas of time and space, and finally an encounter with a spongy, green tentacled beastie that absorbs his consciousness into its own.

And odd little tale, highly reminiscent of Long's "The Hounds of Tindalos", but not nearly as effective.