Metrion, also known as Lord of Undeath and Bringer of Necromancy, is a Reaper. He is believed to be one of the three Great Reapers of Death with full control of Necromancy. Metrion played an integral part in bringing Necromancy to the Mortal Plane as well as his participation in the events of Darkfell. It is unclear how he entered this plane during any of his visits or how exactly he was banished each time. After the Battle of Amerale, he was forced back to Afterlife Realm by some means.
Currently, Metrion possesses a version of himself on the Material Plane, composed of his will and influence. While his true self remains trapped, his Aspect resides on the Material Plane. His Aspect is significantly weaker than himself and can be killed as if a Lich in most regards, however he is quite powerful compared to most regardless. This version of himself controls the Death Knights.