Black jellies are amorphous creatures that live only to eat. They inhabit underground areas throughout the world, scouring caverns, ruins, and dungeons in search of organic matter, living or dead. They attack any creatures they encounter, lashing out with pseudopods or simply engulfing opponents with their bodies, which secrete acids that help them catch and digest their prey.

The typical black jelly measures 10 feet across and 2 feet thick, and weighs about 10,000 pounds. Black jellies of smaller sizes may be encountered, possibly as a result of the splitting described above.