Ahriman: Undying by John French

Ahriman languishes on the threshold of space and time, adrift in a sub-realm of unknowable dimension and aspect. He has found it at long last: the ancient device known to the necrons as the Key of Infinity. This should be his moment of triumph - but he is not the Ahriman he used to be. Betrayed and lost, he is a solitary sorcerer, bereft of allies, standing amongst the smashed and ruined ephemera of his grand designs.
With his dreams as dust and his Legion consumed by the fires of the Pyrodomon, Ahriman must fight his way across the shattered remnants of his past if he is to have any hope of saving his future.