Archmagos by Guy Haley

Belisarius Cawl has an audacious plan to create a stable route through the Great Rift into Imperium Nihilus, but completing his work requires secrets hidden on an ancient Necron world trapped near a black hole.

Follow one of the Imperium's greatest minds as Cawl recruits allies from across the Adeptus Mechanicus and finds himself drawing the attention of forces far more dangerous than he anticipated.