On the surface, Will Graham hints at the existence of ghosts. There tend to be two kinds of horror stories at their most base level, those that feature supernatural threats or concepts that allow for events to occur that never could in reality, and those centered on spotlighting the terror to be found behind the mask of humanity. For some, monsters and demons are a scarier thought, but for others, it's much easier to get scared by a scenario that could conceivably happen, as serial killers are sadly not a rare thing in real life.
Hannibal, as well as the Thomas Harris franchise that spawned it, is strictly an example of the more realistic brand of horror, at least in the sense that a man with Hannibal's intelligence and dark proclivities could conceivably lurk out there at any moment looking for victims. It's unlikely he'd be able to get away with stringing up a detective in a public square or cause his lead pursuer to fall in love with him, but this is still fiction after all.
Still, the last thing anyone would expect to see on Hannibal, or in a Hannibal movie or book, would be a ghost. Yet, it seems to be present in one of the most recent adaptations of the beloved literary character, given life by Mads Mikkelsen. According to one fan theory that has been doing the rounds on the Internet, there is sufficient evidence to the notion that there are in fact ghosts present during season three of Hannibal.
At the end of Hannibal season 2, Hannibal and Bedelia escape to Italy, leaving behind the bloody bodies of Will, Jack Crawford, Alana Bloom, and Abigail Hobbs. Season 3's premiere centered entirely on Hannibal and Bedelia's new lives abroad, but the next installment seemed to reveal that both Will and Abigail had indeed survived their horrific treatment by Hannibal. That's until the end of the episode, which shows that Abigail died after Hannibal slit her throat, and Will has been struggling to process his grief and trauma stemming from the events of the season 2 finale ever since.
It would be easy to dismiss Abigail accompanying and interacting with Will throughout the episode as a hallucination or a daydream conjured up by Will's damaged psyche, but at one point when the two are in a church, Abigail and a priest exchange a quick glance, then turn away. Since she was already dead, that must mean her ghost was literally with Will for much of the episode, if another person could see her. However, it's still ambiguous, as no one else seems to see her, and the priest could've been looking at Will from an angle.
For his part, Hannibal creator Bryan Fuller later confirmed that he intended to leave the question of whether Abigail's ghost was actually there up for debate, leaving a supernatural mystery for fans to ponder. If ghosts really do exist in Hannibal's world though, one can only imagine all the spirits who've paid Dr. Lecter himself not so friendly visits over the years. It also makes one wonder though if any of Will's prior visions or nightmares during cases were just due to trauma, or were connected to something more otherworldly.