According to a dark prophecy a Maia will descend from Heavens to aid Morgoth in breaking free from the Doors of Night . He supposedly will be a Demon of the Void equal to Sauron. A powerful being, he sets forth during the last days of evil in the battle known as the Third Song or Dagor Dagorath.
Fanfic:The Story of the Endless Dream, on planet-tolkien.com
Gilthrang is not featured as a character in MERPS/ICE, though Gamemasters may wish to use his influence and create a unique gaming experience. He is considered as powerful as Sauron, (which is left up to interpretation) and should not be included in Games directly as he is biding his time to help release Morgoth from his imprisonment.