Essences can be seen as a pity timer for untiered items. Essences are limited to enemies that drop untiered items. Killing one of these enemies will have a chance to discover its Essence. Thereafter, killing one of these enemies has a chance to drop its Essence. You can collect up to 3 different essences at a time and exchange all of a certain Essence for the untiered item you desire by speaking with Astake, the Witch Doctor in the Tavern.
Discovering a specific Essence is as simple as killing that specific enemy until you discover its essence. Upon discovering an essence, it will flash the essence and its name across your screen.
To begin collection of an Essence, you must speak with Astake, the Witch Doctor. Astake can be found above the vault doors in the Tavern. Interacting with him will allow you to choose up to 3 different discovered essences to activate and collect. After that, kill the enemy for a chance at its essence.
To exchange a stack of Essence, speak with Astake and, using his black magic, he will assemble the Essence into the untiered item of your choice. These untiered items are tradeable just like any other item.
Note: Exchanging a stack of Essence will consume the entire stack. For example: If an item costs 35 essences and you have 50, all 50 will be consumed during the exchange process.
These are the currently available Essences and where they are dropped.
The current Essence Exchange rates from Astake, the Witch Doctor.