Any monster in the game can drop a Soul Well. Whenever you clean an area, all souls you killed are captured in the Soul Well. Once you have collected enough areas, you can open the Soul Well to visit the The Phantom Reliquary where you can defeat all the souls again. If you succeed, Urgrim appears. He is a grim reaper. If you defeat him, he drops Reaper Harvest, a special potion that can receive a variety of effects, although with long cooldown.
The full change log is here:
https://www.dropbox.com/s/g8deyhsrg73c2an/Changelog.txt?dl=0