As he was a man of great stature and influence, the Tenth Doctor was surprised to find him assisting Inspector DuPont in the investigation concerning the Vampire in Paris in 1895, and living in the bohemian quarters of the city instead of an apartment in the posh Place Vendome area, for which he held title deeds.
The Doctor was right to be suspicious. De Guerre was one of the vampires first victims, having been turned to bones. Varlos, who arrived around the same time, had assumed his identity. (PROSE: The Vampire of Paris)