Manius Grays was once a member of the Temple of Vanus roughly 600 years ago. He abandoned the church because he began to feel that the underground was more of a prison than a sanctuary. In a desire to force people to see otherwise he sought to force people to the surface and there align with Asmodius, who he saw as the only person powerful enough to ensure their safety.
Manius was put down by the Temple and the Royal Mages. He was recently resurrected by a misguided group of clerics in the Temple.
He is a known Psion, and likely a Thrallherd.
It seems he has began efforts to exert his will on the world once again. He revealed that he has established himself as a powerful merchant and trader, which is likely how he has been able to easily move things between the surface and the underground.
Manius is charming, intelligent, cunning, and able to control the wills of others. He is also almost completely insane. His own abilities, in connection with the power and resources he has gained, make him an extremely dangerous enemy.