"The Thing in the Pit"

by Lin Carter
originally published Lost Worlds, DAW 1980

Zanthu, high priest of the cult of Ythogtha, hope to revive his fading religion by overcoming the rival cult of Ghatanathoa once and for all.  This has been tried once before - and failed.

His researches lead him to a ritual called "The Key That Openeth The Door To Yhe", a highly dangerous ceremony, but one he feels he must undertake.  

He journeys to a secret temple deep in the bowels of the earth, and there summons Father Ubb, a big drooling worm-thing.  There he learns the last bit of secrets, and thus sets out to perform a ritual to summon Ythogtha.  That turns out to be a big mistake, as Ythogtha is pretty gross, and he ends up sinking the continent of Mu, though Zanthu and some acolytes escape.

I would call this a pretty minor Clark Ashton Smith pastiche - and that it is, with prose so purple even CAS would blush.  But it does almost have that Smith-ian smirk to it.  I mean, it's not really a good story, but it is fun to read.