Though found commonly in underworld locations these are considered fleshy creatures and are susceptible to conditions such as bleeding, poison, etc.
Each of these bosses drops up to three (3) items.
Make sure to be ready to pick up those items and have room in your inventory for them. When the last monster is slain a countdown of 30 seconds will start, after which you will be transferred back to the Tomb of the Primeval Kings outpost. If you haven't picked up your drops by then, don't panic, they get put in the unclaimed items box and you can take them out of it once back in Tomb of the Primeval Kings.
A better tactic is to let one group in the area live, kill the Darknesses, get your drops and clean the area after that. The timer will only start after all the visible groups on the map have been killed.
These Darknesses are a reference to the Dead Alewives' Dungeons & Dragons skit, as seen when announcing that you are attacking them via Ctrl: "I'm attacking the Darkness!" (see also Ogre-Slaying Knife and Galstaff the Light).