In Curse Of Strahd, a group of adventurers called Pack investigated the orphanage, and found out Felix's secret. They battled his demon in the attic, and eventually managed to slay it by destroying the locket, which returned Felix's control over his body. He was severely traumatized by these events, as he had retained consciousness for most of the possession, but slowly got a sense of his old self back.
A troubled resident of Saint Andral's Orphanage, Felix had no interest in fitting in at the orphanage or speaking to other children, and caused problems with its daily operations. He had lost his parents at an older age than most orphans, as they died in a "bloody accident" when he was nine. The truth of the accident, however, was that the boy had killed them both in their sleep with a kitchen knife.
Unknown to anyone, a Shadow Demon had possessed Felix through a cursed locket that he stolen from his father's nightstand. This Demon latery caused him to attack Zaharia Lucivich, a bully at the orphanage, pushing from an open window to his death. It would influence Felix to carve Infernal runes in the bedposts of other orphans, then used the runes to channel magic that allowed it to feast on their nightmares. This magic manifested in bruising and wounding on many orphans, such as Tessa Nicoleva, and placed a deathly curse on Milivoj Temesvari.