The Monk's TARDIS disguised itself as a Saxon sarcophagus in order to fool the Saxons. It belonged to the Meddling Monk and was used to store the items and plans that he had found and created during Earth history. When investigating the Monastery, Steven Taylor and Vicki Pallister came across it to their surprise.
After deciding that the Monk would be trouble, the Doctor decided to remove the Dimensional Stabiliser, causing the Monk to be trapped in his own timeline. (The Time Meddler)