Demogorgon was a patchwork abomination made up of shapeshifting creatures, designed by a mad Chaosian mage, Ulmno the Mad, with the purpose of destroying the Logrus; as it ate, it grew larger and larger. Long ago, it was hacked into pieces with Logrusfoils after a great battle, and the pieces were dumped into the Abyss.
Recently, due to the great disturbance in the Abyss that occurred due to the destruction of the Logrus's extension in another universe, a part of Demogorgon was cast loose from the Abyss, bearing within itself the Gloom Device, the machine responsible for the destruction of the Logrus in that other universe--with the potential to destroy the Logrus of the UnS universe as well.
Demogorgon roamed through Shadow, devouring others and growing stronger, hunted by the forces of General Gilva, and by Motoko, Moriya and Davros. He was finally discovered by those latter three, by which point he had grown to the size of Godzilla, and was, in fact, in combat with Godzilla when they found him--shortly before a random activation of the Gloom Device caused him and the King of the Monsters to fall in love with one another.
After a long battle involving hundreds of Chaosian troops, horny bison, beehive hairdos, and the mind-controlled Godzilla tearing into Demogorgon while Demogorgon was blasted by spells and missiles, Demogorgon was torn to pieces and destroyed for good by Davros's spikard. Before he died, however, the last activation of the Gloom Device killed nearly all of the Chaosian troops, leaving only the strongest among the attacking forces alive.