Heroes of Loot

Lost souls are caged humans and are randomly found in dungeons, they don't start spawning until around dungeon 10 and after the first dungeon difficulty upgrade. You will ultimately come across dozens of these as you work toward 5000 kills. Many higher dungeons contain several cages. Shoot the cage to free the soul, earning the trophy and usually getting a helpful item as a reward (magic, health etc.).

This is what a Lost Soul cage looks like: