Paradox is a time manipulating being of unknown origin. He is often spoken of in the myths and legends as a non-existent being that even gods couldn't defeat.
The first records of Paradox's existence was during the Time War when a meteorite fell to Gallifrey. Scientists studied it for months before they all died of mysterious deaths. The meteor soon cracked open and released the imprisoned Paradox, who disappeared for several months. Lord Rassilon was confronted by Paradox who made a deal, if he put the whole of Gallifrey in a Time Lock, he would give him infinite strength. So several hundred years later, Gallifrey was put in a Time Lock, but a burst of energy shot out, an unknown force which hit Paradox, who disappeared.
Paradox found himself at the centre of the Moon. Unable to use his powers, Paradox lay at the core of the moon for centuries. Slowly, the care of the moon started fading away due to Paradox's time-distorting powers. He waited another century before he was able to escape. Paradox escaped and went to the Medusa Cascade to regain his power.
He then used his powers to build a palace in the Medusa Cascade. He genetically altered several Whifferdils to do his every command. The Whifferdils took the form of Judoon and Paradox set to work building his Crystalline Power Amplifier and his Universal Collapse Generator. He had come up with the ultimate plan and nothing could stop him, except maybe the Doctor.
Collapse of the Universe
Paradox activated the two machines and a shock wave shot across time creating vortexes all across the universe. The vortexes would soon disappear, but the Universal Collapse Generator sent out a blastwave that destroyed everything in its path. The Universe ended leaving nothing but Paradox's palace. Amy however tricked Paradox into an airlock, where she opened the door and the two were sucked into space. Paradox was caught in his own blastwave and supposedly destroying him along with the rest of the universe.
Key life events