Marcus Mason is the son of Jeremiah Mason and the brother of Darien Mason. While Darien was the control for Jeremiah's genetic experiments, Marcus was the end result: a synthesis of Jeremiah's own genetic code and that of their ancestor, the demon Bloodwing Dragonash. When his abilities surfaced, he proved to be too powerful for Jeremiah to control, so he had his trusted servant murder him. When Jeremiah ordered the death of that servant following the New Erebus mission, the servant reanimated Marcus as a last act of revenge.
When Marcus awoke (still as a boy when he died) in the family crypt outside of Baltimore, he lashed out at the nearby city with his powers in blind revenge. The surviving demon-ship from Bloodwing's fleet, the Midas, tracked his path of destruction and took him aboard as heir to the Bloodwing dynasty. Marcus was prepared to use the Midas to attack nearby Washington City and conquer the United States when the Mason family (including Aleister Mason's Blood Doll army) and Caledon's Wrath Fleet intervened. The Midas retreated to Winterfell airspace.
The details of the Midas' destruction are still secret, but Marcus escaped. He reappeared on the island of Belhaven after its ruler tried to summon Bloodwing in a botched ritual. Since Bloodwing was already destroyed, the magics summoned Marcus instead. In payment for saving Belhaven from the Hydra, which tried to invade the Earth through the malfunctioning demongate, Marcus forced the Dowager Countess Eve to bestow him the title of Count Belhaven and make him co-ruler of the island.
Marcus began a physical transformation, taking on the wings and other distinctnve traits of Bloodwing. He was promptly decapitated by a Qlippothic Steel unit, signaling Jeremiah's return to Earth and a new war upon his progeny. His body is in storage in Mason Labs. His head, however, is still missing.