From publisher blurb: The denizens of the lower planes are among the most dangerous foes any hero could encounter. When they are highly trained and have lived been terrorizing the planes for thousands of years, the threat is more challenging. Five of the most destructive creatures of darkness are gathered together as the Formidable Fiends. From an erinyes bounty hunter to a marilith relishing the slaughter of innocents, these enemies will provide a suitable challenge to even the most powerful heroes. Ready your weapons and spells, the Formidable Fiends await!