"The Scroll of Morloc"

by Lin Carter
originally published Fantastic, October 1975

Yhemog is a shaman of the Voormis.  Pissed-off after having been passed up again for high priest, Yhemog decides he is now an atheist, and wants to stick it to the worship of Tsathoggua.

The Scroll of Morloc is an artifact housed in the Temple of Tsathoggua, sacred to the rival deity Rhan-Tegoth and his followers, the Gnoph-Keh.  Yhemog decides to take the scroll and perform the ritual in the temple, thus to defile it.

He gets into the temple, frees the scroll, performs the ritual, and finds himself transformed into a Gnoph-Keh.  Temple guards, finding a Gnoph-Keh in the temple, kill it quick.

Another amusing C.A. Smith pastiche.