For years, Belisarius Cawl has nurtured an audacious plan to stabilise the Attilan Gap sufficiently that the Imperium might open a stable route into Imperium Nihilus. With the primarch Roboute Guilliman having crossed the Great Rift, and now isolated on the far side, time is running out. Before Cawl can perfect his latest great work he requires one last element: control codes preserved on an ancient necron world trapped on the event horizon of a black hole. Realising that even he cannot do this alone, Cawl recruits help from across the Adeptus Mechanicus. Yet as victory nears, malign intelligences have taken note of the archmagos, and Cawl is about to discover it is unwise indeed to provoke the interest of Vashtorr the Arkifane.