Any time a Carcosan Sorcerer makes a ritual human sacrifice the DM rolls 1D6. On a 1, the soul of the sacrifice doesn’t quite shuffle off their mortal coil. Perhaps the sacrifice was clumsy, or they took too long to die, or made eye contact with the celebrant at the wrong moment. The exact reason is both irrelevant and up to the DM. All that matters is the haunting.
The lost soul is now intertwined with that of the Sorcerer (PCs with Divination or Astral abilities can actually see a shape trailing behind the haunted Sorcerer, holding to his shadow as a child clutches the hem of a mother’s gown). Every day for the rest of the Sorcerer’s life, the DM makes a secret D6 roll. A result of 1 means the soul has manifested enough energy to hex the Sorcerer that day (usually this means an unexplained -1 penalty to an important roll the Sorcerer makes that day, but other misfortunes are possible).
DMs may wish to keep the haunting an unexplained phenomenon or curse until a dramatically-appropriate moment of revelation, or may simply choose to treat the haunting as an adversarial NPC who teases and mocks the Sorcerer, keeps him from sleeping, etc. Griffin Dunne’s character from An American Werewolf in London might serve as inspiration for haunts such as these.