Ryuu no Otto: Boukoku no Kami

Upon his return from battle and a strange encounter with a monster-like man, The Captain is notified by his twin brother, The King, that their Guardian Deity no longer wants to protect the kingdom. To convince the Guardian Deity to change its mind, The Captain ventures out to its mountain lair and returns with the Guardian Deity's only condition to continue protecting the kingdom: The Captain must become his mate.
(Source: Futekiya)
Note: Contains an epilogue chapter.