In March 1215, de Lacy went to London to swear the Crusader's Oath before King John of England. On his journey back to Fitzwilliam Castle, he met with Sir Giles Estram — in reality, the Master — who had him arrested and taken before Kamelion, who was in the guise of the king.
The King freed him in favour of executing Estram, but the Fifth Doctor ordered him taken down to the dungeons. This was a ploy so de Lacy could show the Doctor the way. Once they arrived, he was freed. The Doctor told him to return to London and warn the king. As he rode from the castle a guard, acting on the Master's orders, shot him with a crossbow. He was returned to the castle where he uttered the words, "The King... Doctor... Seek", before he died. (TV: The King's Demons)