Ember Dragon

It's good. Real good. Lots of cool monsters and lore. There's a content warning for a reason, but if that doesn't bother your table, go ahead and buy it. The DM can make tweaks to it if it does. (For the religious folk out there, this book is for a harmless game of make-believe called D&D. It doesn't actually let you summon demons or anything like that. In fact, it's so that the players of D&D can know what sorts of fictional demons they can have their fictional characters battle against in their games.)