Utilize

You can use the Utilize action to attempt to harvest a specific essence from a creature that died within the last round. Make a survival check with a DC of 10 + the creature's CR. On a success, you harvest that essence. On a failure, the essence is destroyed.

As the GM it's up to you to take a look at the creature's monster statblock and determine what useful parts there are and describe the player's success or failure. Alternatively, you can ask your table to contribute any ideas.

For example, Ankhegs have Acid Spray. Altan the Wizard makes a successful arcana check, identifying that these glands could be used as an essence in the creation of a Potion of Resistance or Armor of Acid Resistance. When the Ankheg dies, he calls out to Maeve the Ranger, telling her to cut out it's acid glands.

You can also use a creature's CR as a guideline in determining what rarity of magic items and essence from that creature could be used to create.

This is useful as part of the The Arcane Forge rules