A Paradox machine was a device made from a TARDIS to allow time paradoxes, such as allowing the Toclafane to kill their ancestors without any temporal consequences.
The interior of the paradox machine was lit by an ominous red glow. While it was active, the cloister bell sounded continually.
The main console in particular had undergone many modifications by the Master: The console itself was enclosed inside a metal cage, with tubes and appendages attached to it. At its base was a pipe connecting to another area of the TARDIS, with a dial showing a readout of the pressure build-up inside. It shot out puffs of steam when it was activated.