The Bestiary is a feature in the game that at the start lets you see the base stats of every monster you have encountered so far (HP, armor, resistance and EXP). You can find it in the “Bestiary” section of your guild menu (default key is "G").
However, the Bestiary can become a new way to augment your runs similar to the Magic Anvil. There is an NPC named Baltzar that will initially appear outside the Guild Hall. If you speak to him he will tell you that he lost his Book Of Monsters in the Forsaken Tower’s Archives, and asks if you can get it back for him. Similar to the Magic Anvil, you have a chance to find the book while playing on NG+0 (normal mode), and a near certain chance to find it while playing any higher level of NG+.
Note that the Book can be found within rooms revealed by pressing the button in the Archives and is marked with a purple beast icon on the map.
After you find the book and return to town, you will find Baltzar inside the Guild Hall now, with his book in front of him. If you go speak to him now, he will open the book and a new upgrade menu will open. In this menu you can scroll through every monster you have encountered and see their base stats like before, but you now also have the option of spending some Skill Stars to attune yourself against every single monster you have encountered. Attuning yourself against any monster will permanently increase damage done to them by 20%, and permanently reduces any damage you take from them by 5%.
You can also attune yourself against the same monster multiple times. Every further attunement will increase your damage dealt to that monster by 20%, and your damage reduction against them by 5% at early levels, however the damage reduction has a diminishing return value, and so at higher levels you will receive less than 5% damage to your damage reduction (this is to keep things more balanced). The cost of attuning will also increase each level by the same amount the attunement initially cost (ex. For Legendary monsters the initial cost is 10 stars, then goes up to 20, 30, 40... etc.)
Like the Magic Anvil and your character’s guild hall skills, these upgrades can be reset with a full respec for a cost of 250 gold per star you have invested. This will allow you to change what monsters you have attuned yourself against. One potential strategy you can apply with this is to only attune Forsaken Tower enemies when playing the tower, and only attune Pyramid of Prophecy enemies when playing the pyramid (note that obtaining the gold required to do this can become costly).
Monsters are divided into 5 classes, similarly to how items are divided:
- Legendary: Final bosses, 10 Skill Stars to attune yourself against
- Epic: Non-final Bosses, 8 Skill Stars
- Rare: Mini-bosses, 6 Skill Stars
- Uncommon: Elite monsters, 4 Skill Stars
- Common: Normal monsters 2 Skill Stars