Mammon

When Joe and Grace Callahan discover they belong to a secret order of mercenaries, they join an ancient struggle between their own kind and the once-human Earthborns, seduced into evil by their leader, the demon Mammon. Joe is a Ferryman, with the rare power to open rifts into other dimensions. His sister, Grace, has rapidly growing telepathic powers. Their gifts fascinate and terrify those around them, and the siblings are set to become the most influential players in the war.But Mammon grows stronger every day, and now he wants Joe to open a dark rift to the demon world, where a Hellborn army is waiting.