Probably time
Shooting darkness into people and corrupting them to serve you
i don't think they should be outcasts. They are more powerful. And it shouldn't be two elements, it should be a mix between them. If there a Fire and an Earth marry, their child(ren) have a chance of being fire, earth, or magma.
He can temporarily or permanently make something turn into a ghost. Like, he could make someone a ghost, then put something inside them, and unghost them. The weapon would be lodged in their body. He could also fly and go through walls.