Supernatural's explosive season 14 finale, the Winchesters and Castiel were left reeling by the death of Jack, who had been given the smiting treatment by God himself. Before the impact of Jack's demise could fully sink in, the boy's body was used as a vessel for a demon calling himself Belphegor.
Belphegor claimed to be a pen-pusher in Hell, dealing in the torturing of souls cast down to the fiery side of the afterlife. The demon said he loved his job, and was as upset as Sam and Dean about God's actions, resolving to help them round up all of the rogue souls that had escaped and send them back into the depths. After an understandable outcry from Sam, Dean and Castiel, Belphegor explained that he only jumped into Jack's body because it was the nearest one to the hellish rupture that wasn't a wormy corpse. Despite harboring reservations, the heroes decided to work alongside Belphegor, but they might've been better served listening to their gut instinct.
In demonic mythology, Belphegor is actually a Prince of Hell. If he boasted the corresponding title in Supernatural, then that would put the demon on the same level as Asmodeus, Dagon and the Winchesters' arch enemy, the yellow-eyed Azazel, all of whom have been major final antagonists in previous seasons. It can also be assumed that Supernatural's Belphegor is based on the established figure from lore, because the character mentions worshiping a phallic rock in a past life - something that followers of Belphegor also did according to the real-world legend.
Another damning piece of evidence is Belphegor's decision to use Jack as a vessel. Thanks to God's thorough smiting, Jack's eyes were burned out of his body. This is mightily convenient for Belphegor, since eye color is a dead giveaway for demons. While regular demons (like the pen-pusher Belphegor claims to be) possess blackened eyes, Princes of Hell have yellow ones. This would be ring major alarm bells for Sam and Dean, but possessing Jack allows Belphegor to avoid the problem entirely. Interestingly, the demon promised to transfer into a new vessel as soon as possible, and is yet to do so despite a number of fresh corpses appearing over the past two episodes.
If Jack's body was possessed by a Prince of Hell, this would explain why Belphegor has been able to use such powerful magic. After first appearing, the demon used a simple mixture of graveyard dirt and angel blood to repel dozens of souls from attacking the Winchesters. Later on, Belphegor used some regular salt and a human heart to create a ghost barrier around an entire town. This is incredibly powerful (and convenient) magic for an everyday demon to know about and the required ingredients always seem so readily available. It's possible that Belphegor is merely ing off his own powers as spells to avoid suspicion.
Lastly, a fellow demon called Ardat hired Arthur Ketch to assassinate Belphegor, claiming he posed a threat to the entire world. Belphegor manages to successfully brush this off as a petty spat between two demons, and Sam, Dean and Ketch all buy the explanation. Considered alongside the other evidence, however, it seems highly likely that the threat Belphegor poses is all too real.
But what would a Prince of Hell have to gain by working with Sam and Dean? Perhaps the demon's motivations are genuine, and God's disruption of Hell's business has irked Belphegor. Alternatively, his true goal could be to manipulate the Winchesters into collecting up the souls of Hell to use as a power-up later on, and Rowena has already started hoovering them up with her crystal. Dwelling within the vessel of a Nephilim such as Jack, Belphegor might just be the most powerful demon Sam and Dean have ever come up against.
Supernatural season 15 continues October 24th with "The Rupture" on The CW.