While Horus’s uprising burns the Galaxy, another war is underway in the undercrofts beneath the Imperial Palace. The Custodian Guard—also known as the Ten Thousand—together with the Sisters of Silence and the army of techno-priests of General-Fabricator Kain fight for control of the key nodes of the ancient Eldar Web closest to Terra against demonic entities that broke through the tunnels following the path of Magnus the Red. But now that the Traitor Legions and defiled titans have joined the forces of Chaos, the noose around the Throne World begins to tighten—and no one but the Emperor himself will be able to win this battle.