Cloister of the Frog God

From the publisher blurb:

The cloister has stood on a desolate ridge overlooking vast swamps since time immemorial. Dedicated to the great and terrible Tsathoggus, this edifice of evil was destroyed again and again through history, only to re-emerge from its slumber once the forces of Law grew lax and the terrible deeds of the frog-cultists became forgotten. Now a new order rises among the old walls, while older evils stir in stone grottoes and underground sanctuaries. Spies visit the settlements of the marshlands, and offerings make their way to the cloister where the monks hold their vigils as their ancestors have, guarding a nightmare that refuses to die.