Castor by Jaclyn Osborn

When Castor was captured by demons and forced to fight for his life in the underworld, he never expected to be rescued by a water dragon wielding a katana. He also never expected for that dragon to turn out to be his destined mate.
Too bad he has no time for romance right now. With the son of Lucifer building an army, Castor and his half-angel brothers are busy trying to stop the end of the world. But his attraction to Kyo is magnetic, and as the personification of Greed, he’s never been good at denying himself anything.
A trip to Greece brings Kyo and Castor closer in more ways than one, and neither man can fight their soulmate connection. But with a war on the horizon and enemies at every turn, will that connection be broken?