You've got heroes in your RPG. Unless you plan on having them just sitting in the tavern, sipping ale and not doing much of anything, you'll need some monsters for them to go and attack. It's sort of what heroes tend to do. But what sort of monsters will they go fight? Paizo is helping out with that last one. They're working on Bestiary 6, which will bring not just ordinary monsters to your tabletop, but also some rather legendary ones as well.
What sort of legendary monsters? How about Mephistopheles? Though before you get to him, you'll probably meet Charon. If you've been naughty, you might fight against the Krampus. Just all sorts of stuff going on there. The book will also have other, lower-level and less-legendary monsters. There'll be more than 200, in fact.
For players, there's going to be new animal companions as well as playable character races.
Expect the book to hit store shelves in March of next year.