Diomin is a dark fantasy role-playing game, designed by R. Hyrum Savage and published by OtherWorld Creations (OWC).

Diomin was a fantasy setting which was originally designed for GURPS, but after starting RPG publisher OtherWorld Creations (OWC) during the d20 boom, R. Hyrum Savage and his friends Chad Cunningham and Chris Miller were convinced by the d20 licensing to convert Diomin to d20 and the company released it in 2000.

OtherWorld supported Diomin with publications through 2003, but production was halted soon after with the crash of the d20 market.

Savage brought back OtherWorld Creations in 2006 using another licensing opportunity similar to the OGL, so Diomin was brought back for the Mongoose Publishing version of the RuneQuest system, with new books that were mostly released as PDFs.

The Paladin class receives the most in terms of game mechanic changes, followed by the Wizard.