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The Crucible was the flagship/superweapon of the Dalek Fleet of the New Dalek Empire at the heart of the Medusa Cascade. It was a mostly spherical space station the size of a planet, with numerous docking ports on the outside. It also had a unique core of Z-Neutrino energy which powered the reality bomb.
During the War in the Medusa Cascade when the New Dalek Empire invaded the captured Earth, the Crucible was at 90% efficiency. The Crucible had control over the entire Dalek fleet. (TV: The Stolen Earth) Human prisoners were taken there for the testing of the reality bomb.
The Crucible was to be the staging post for the deployment of the reality bomb, which would have destroyed all of the universe except for the inhabitants of the station. It was destroyed, along with most of the Daleks, by the Tenth Doctor's clone. (TV: Journey's End)
The Crucible was a huge sphere. It was split into two halves by an equatorial rim, from which extended at least five arms. At the ends of these arms were large docking stations for Dalek saucers.
The exterior of the station also contained large circles which may have been part of the reality bomb. Long, unexplained lines lay on the hull. Beneath the surface, various concentrations of light could be seen. The hull contained at least one circular shaft leading to the bridge. The TARDIS, immobilised by a temporal prison, was pulled from Earth to the Crucible and along this shaft to the bridge.
The interior of the Crucible contained the colonnades typical of Dalek saucers. The bridge consisted of a metallic platform and a large door resembling a blast door which opened into another section of the station where the Supreme Dalek seemed to reside. This door opened onto a raised platform on the bridge from which the Supreme Dalek could address the Daleks. The bridge seemed to be directly above the Vault, as the Supreme Dalek could descend straight from his "throne" into the Vault. The shaft was covered by a trap-door-like mechanism which could be opened to deposit objects into the core. The bridge platform was located inside a larger chamber where at least dozens of Daleks could hover.
The Vault was a chamber which housed Davros and Dalek Caan. Elsewhere on the Crucible was a section built of some sort of ceramic material. Human prisoners were brought here for the testing of the reality bomb.