Abaddon's key was not a metaphor. Drau truly possessed a key that could control the Degraded Ones; once activated, it could make them disintegrate in batch after batch.
But Drau never felt it was safe. After the Battle of Cadia began, the Dark Mechanicum had extensively modified and upgraded the Degraded Ones. Who knew what kind of twisted abomination they would eventually create?
Once Cadia fell, Drau would collect his payment from Abaddon and then find some remote star system to continue his research projects.
Under the attacks of Warp storms and Degraded One swarms, the Imperium fleet defending The Rock suffered defeat after defeat. Even The Rock itself became infested with vast amounts of degraded debris. If not for the united efforts of the Dark Angels Chapter and its successor Chapters, this enormous fortress would have become the vanguard of the Chaos forces.
The disordered Imperium fleet attempted to withdraw in order to preserve its remaining strength, but how could Abaddon possibly let them have their way? The winds of Chaos spilling from the Eye of Terror provided fertile ground for blasphemous sorcery. Wrecked warships were dragged together and assembled into an array found only in demonology, then activated with countless souls.
In an instant, the Deathmatch Cage trapped both the Chaos fleet and the Imperium fleet. Until a victor emerged, no one could leave from within.
Yet the spell only stated that those inside could not leave. It never said that people outside could not enter and meddle.
The Warp convulsed violently several times, and neither Chaos nor the Imperium had caused it. Every ship's sensors detected vast numbers of colossal constructs dropping out of Warp transit and entering realspace. Though no one had yet made out the outlines of those gigantic constructs, the earthshakingly savage cries alone