He had allied himself with the Ants. They brought him treasures from throughout history to pay him for his cooperation. When the Seventh Doctor came through a space-time rift to Thierry's estate in 1871, he allowed the Ants to try extracting the Doctor's knowledge of time travel, but they failed.
Eventually deciding the Doctor had been handled too mildly, Thierry threatened him at gunpoint. He used a device to bring Kadiatu Lethbridge-Stewart's time machine to them, telling the Doctor he would activate it to open a rift to the Ants. When they arrived, they would use the machine to create more rifts, stabilise them, and loot anywhere and anywhen they chose.
However, the Doctor had rigged her time machine to explode when it was moved. He had hoped it would be taken by the Ants back to Ship and destroy it. Instead, it imploded on Thierry's estated, sucking him in and killing him. (PROSE: Set Piece)