While Horus’s uprising burns the Galaxy, another war is being fought in the dungeons 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 the general-fabricator Khaine, fights for control of the key nodes of the ancient web of Eldar closest to Terra against demonic entities that have broken through the tunnels following the path of Magnus the Red. But now that the traitor legionnaires and desecrated 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.