Obsidian, Born of Stars and Darkness

An eldritch being from a strange, demonic-ravaged plane. Obsidian was able to cross from his realm to Aeridos through a miraculous stroke of luck. Ben Aldon, a human who had been cast through a portal into the jaws of a Greater Demon, became the bridge that allowed Obsidian to escape his hellish world. Attracted to the mortal’s magical essence, Obsidian fused with Ben’s body and rescued him by returning through the portal to Aeridos. Unable to live without a mortal body to inhabit in the mortal world, Obsidian set to work repairing Ben’s brutally ravaged body and learning as much as he could from his mind. He let himself remain physically chained, a captive of a cult who had tried to sacrifice Ben to what they believed was a god, and bided his time.
Obsidian is a relatively young member of his species, and has an unusual appetite for experiences beyond the conquest and bloodshed the rest of his race seem to crave. He initially believed that the body he inhabited would allow him to gather strength in this new world, allowing him to carve out a place of power for himself. However Ben refused to let a strange creature wreak havoc on his homeland. Obsidian was horrified to realize he was not the dominant personality in the body and found himself caged behind Ben’s eyes. It didn't take long for Obsidian to figure out that he could assert control briefly while Ben was unconscious or sleeping deeply.
Over time the pair developed an uneasy partnership. Ben's isolated work as a Ranger kept Obisidian far from the cities where he might break Ben's control and cause chaos.
Obsidian's abilities complement Ben's own magic in many ways, making them what Obsidian calls a "perfect bond". When Obsidian and Ben work together in battle they become a terrifying force. Obsidian grants Ben access to the dark energy of his home which enables them to forge temporary weapons. Ben learns to teleport short distances, but must be able to see where he is going, and they are able to surrounded themselves with a carapace-like armour. 
Though the two often vie for dominance in their relationship, Obsidian is often surprised by how much he enjoys Ben's world.