He was an expert sportsman who hunted and collected rare creatures, including Mmtefl Beasts, Zargregs, Moogs, Daleks, a Raston Warrior Robot and even an Eternal, all of which he killed and placed on display in a large trophy room. His ultimate trophy was to have been the Tenth Doctor, the last Time Lord, and he forcibly recruited the Endangered Dangerous Species Society to hunt the Doctor for his amusement.
His plan failed when the Doctor switched places with Baris, gained access to Planet 1's computer systems and, with the assistance of Planet 1 itself, was eventually able to escape. As Planet 1 had decided it didn't need him anymore, he destroyed a large part of its northern continent using explosive decoy robots. Sebastiene tried to hold the Doctor hostage on the condition that he take him away from the planet, but the Doctor refused. Sebastiene was then attacked by the last remaining hunter, the Carpalian Witch, and was last seen fighting it off with his sword alongside Baris. (PROSE: The Doctor Trap)