Astalon: Tears of the Earth

Underneath the very first elevator room (Gorgon Tomb 1), there's a blue door. Enter this door to find a purple bat creature, which gives you your bestiary. You can see this by pausing, then pressing Y.

You need to reach "Completed" 82/84 -- everything except bosses 5 and 6 right at the bottom - and then fight the final bosses. I never actually noticed mine pop, but from the time, it must have been just as I beat its final form.

There's a full bestiary here: https://steamcommunity.com/sharedfiles/filedetails/?id=25088...

Even better, it includes a map showing where to find any enemies you still need to max out. (for me, most were in Ruins of Ash, but your mileage may vary).

The Bestiary and Map belong to Demajen, who does amazing guides for Metroidvanias: https://steamcommunity.com/id/demajen/myworkshopfiles/?secti...

If you want to thank him -- buy him a coffee at https://ko-fi.com/demajen