Demon Lord VIII - When Angels Fall. Demon Lord, #8

The archangel Majelin has been imprisoned and tortured in the underworld of Sherinias' domain for five centuries. Eventually, his light will die and he will fall to the darkness. Whispers of a tar'merin travelling with Kayos have reached Airedene, the Sacred City of Angels, giving Sarmalin hope that her husband might be saved, and she goes to plead with the Demon Lord. The archangel is in a chamber warded against dark gods, however, so Bane must enter it powerless, or Majelin will die.
Bane is forced to use the shadows when demon guardians attack, triggering the trap. He follows the archangel into the underworld's depths to protect him, and they fall into a hidden world inhabited by legendary beings from the Times of Reckoning. Many of Bane's powers do not work there, and his attempts to escape fail, but if he does not return to Sherinias' domain in time, the dark gods he cast down will rise again and enslave the child goddess.
He must find a way out, but the further he travels and the more he learns about the hidden realm, the more he fears that he is in a prison created by a Grey God, and there is no way out.