Bosses and chests. Minibosses drop hand, feet, and waist pieces. Bosses drop chest, legs, head and shoulder. The final boss can give weapons and jewelry. The Adjudicator has a unique dagger.
You can also have a chance to get pages from the Trophy Vault in WGT for key fragments, acquired from IC.
Now, something to know about Essence Thief is that you do have to run to the pool when it appears. So if you can be mobile in the fight, it's alright. If you can't run to the essence, it's less useful.
I personally found it too clunky to use on a regular basis, but your experience might be different.