Belial, aka Pestilence. The name means worthless in Jewish texts.
With no concerns as to his appearance, Belial wears the form of an ever-shifting, disgusting blob, sometimes phlegm-colored, sometimes clear (with evidence of bones and foul debris within). His realm is said to be an endless swamp, hot and thick and crawling with insects, snakes, leeches and other less attractive life forms. His fortress rises up out of the murky grey waters, and is where he devotes himself to the creation of new diseases and vermin.
Of all the Princes, only Malefis has less interest in jockeying for position. Belial is largely uninterested in the politics going on around him; he is content to play the follower and work at his creations in undisturbed solitude. When he does make a rare appearance among the Worlds, it is usually to test some new strain he has created, and an outbreak or epidemic cannot be far behind.