Thorn met Pack in Curse Of Strahd, not realizing he was already dead, and lead them into the Durst Manor. Inside, he tried to possess them into playing with him "forever", but was intimidated by Ghalwin Forrester into submission. They escaped the Death House after killing Walter Durst; though he was destroyed in the resulting fire, the house's Curse remained.
The younger child of Elizabeth and Gustav Durst, Thornboldt was a cowardly but sweet young boy, reliant on his strong-willed sister Rose. He grew up fearing the "monsters" in the basement of the Death House, unaware of his true Durst legacy. He ultimately died of starvation, locked in his room along with his sister, but his Ghost stayed in the Village of Barovia.
The two of them haunted the area in front of the house requesting assistance to kill the monster, unknowingly guiding many adventurers to their deaths.