How do I make one? how do I add creatures to it?

A bestiary is an item in your inventory that can be used to store the memory of one or creatures. Once a creature is stored in such a book, it can not be used as an actual creature anymore, instead if becomes just like a collectors card. These creature that you store in a bestiary, can be converted into NFT on polygon blockchain and taken out of the game, traded, sold, exchanged. Converting NFTs back into creatures might be possible at a later time, but currently they are just collectibles.

 

One place you could get a bestiary item, is from the wish shop, look for Compendium of Beasts item.

I also have the same (second) question what my fellow Ungod asked: How do I add creatures to it?

I have this thought but not sure if it is correct: check the enable box for the "Sacrifice to bestiary" under the Options > Bestiary. Save it. Then to go an Altar, sacrifice a creature, and the creature will be added to the Bestiary item, instead of getting squashed.

Is that it? Thank you for the confirmation.

Once you enable the option, scroll to the bottom of the page for creature you want to add to the Bestiary.

Enter the code under Adanced (sic) Creature Actions and hit Reveal.

You can then choose the Bestiary you want to add it to and add a comment. Then hit Sacrifice for bestiary.

Don't know if it works still, not tried it since the changes made a year or so ago.

Oh man, I should have read your reply here before doing anything else, Ledah. I just did the sacrificing traditional way, directly sacrificed a 3,000 days old creature on an altar, and I was like "What the". 

That process you mentioned still works.

Poor me.