Animus is the magical essence of enemies you’ve defeated with Extraction spells. After gathering specific amounts of Animus, you can use it to craft Treasure Cards that summon those creatures. You can also make treasure cards to banish that particular monster. It can also create Housing Guests, NPC’s you can place in your castle.
When you collect a new creature’s Animus, it will create a profile for that creature. You can see this profile by pressing “T” to view your Monstrology Tome. This profile will show you the School of the monster, whether its a boss monster and its rank. The more powerful the creature, the higher it’s rank.
Higher ranked monsters will need you to have a certain number of Monstrology ranks to extract their Animus. If your level exceeds that of the beast, there’s a chance for you to obtain extra Animus!
As you can see, The Devourer is a rank 18, and it needs 20 animus to create a summon treasure card, 30 to generate a house guest.
Extracting from Mutant Briars won’t be able to take you all the way to max rank. You can only get up to 6 ranks above a mob’s rank, and Mutant Briars are Rank 16, so you can only get up to Rank 22 from farming them. You need to farm the bosses in the Catacombs that are Rank 18 to be able to reach Rank 24.
I recommend farming the Whispering Sepulencher Dungeon because it is easy and in addition to a good amount of extract you also collect reagents for crafting dragoon gear