You will unlock a monster in the library once you kill it for the first time in Vampire Survivors. Once your monsters are killed for the first time, they are added to your Bestiary. Here is a full list of the Bestiary Members in Vampire Survivors, in case some of you are having trouble finding specific entries.
Related: How to Unlock All Stages And Modes in Vampire Survivors
List of Every Bestiary Entry in Vampire Survivors
Right now, Vampire Survivors has 171 creatures, spread across the main game and two DLCs. We have a clickable guide to some of the listed members, and a guide on how to unlock the stage for others. You should check out the following three articles:
