Protagonist becomes the avatar of an alien deity and has to cope with adversary god

Question

I read this at least 20 years ago, and I believe it was originally published in the '60s.

The main character is a human, but somehow "inherits godhood" from an alien, whose species is very long lived. IIRC this is one of the old-style god thingies, i.e. "god of thunder" not "omniscient omnipotent omnipresent transcendent being".

The plot somehow revolved around identifying and fighting some enemy which, I believe, was in turn a member of the alien species who acted as the avatar of the opposite god in the alien pantheon.

The only thing I remember for sure is that the enemy was identified as "green green" or "gringrin" (sadly, I read this in translation so I am not sure what the original meant)

Answer 1

This sounds like "Isle of the Dead", by Roger Zelazny, published in 1969.

... he sought out a mentor, who happened to be a member of a very long-lived and slowly dying alien race, the Pei'ans. Under this tutelage, Sandow eventually became a telepath and "worldscaper". ... Sandow is bound to "Shimbo of Darktree, Shrugger of Thunders" ... it is believed that the worldscaper is actually acting as an avatar for the god. ... Gringrin vowed revenge on the other worldscapers, starting with Sandow. Somehow Gringrin has been able to unite himself with Belion (Shimbo's traditional enemy in Pei'an mythology).