Warning: SPOILERS ahead for Supernatural season 12
The Supernatural season 12 finale may have killed off a whole slew of major characters, but the two-parter also introduced someone new to Sam and Dean's world. 'Who We Are' and 'All Along The Watchtower' started off by dealing with the British Men of Letters - the snooty, monster-hunting threat that has been hovering around since the start of the season. When these monster-killers from across the pond starting taking out every American hunter, Sam (Jared Padalecki) led a contingent of hunters against their headquarters, slaughtering everyone inside. Meanwhile Dean (Jensen Ackles) helped to reverse his mother's brainwashing at their hands, a scene that ended with Mary Winchester (Samantha Smith) putting a well-deserved bullet in the brain of Arthur Ketch (David Haydn-Jones).
With the Men of Letters off the table, Sam and Dean then had to deal with the imminent birth of Lucifer's (Mark Pellegrino) son. In a shocking final battle, both Crowley (Mark Sheppard) and Castiel (Misha Collins) bit the dust, and Lucifer and Mary were exiled to an alternate universe. In the final moments of the episode, Sam heads inside to look for Kelly (Courtney Ford) and her baby - only to find Jack crouched in a corner, fully grown, with glowing golden eyes.
The History of Nephilim On Supernatural
Jack is a particularly interesting Nephilim - he's the son of Lucifer. He has the potential to be incredibly, world-destroyingly powerful... but he's not the first angel/human hybrid to pop up on the show.
Nephilim have been mentioned a few times on the show, although not in a huge amount of detail. It has been revealed that in the past God intervened to remove Nephilim from Earth, and that the birth of a Nephilim is one of the highest crimes in heaven. The penalty for an angel who fathers a Nephilim is death, due to the huge amount of power that these hybrids have. One Nephilim did live to adulthood in the series, though: Jane (Linda Tomassone). In season 8, Metatron (Curtis Armstrong) managed to convince Castiel that he needed the heart of a Nephilim to close the gates of Heaven, and led Cas to Jane. Castiel killed her with an angel blade, inadvertently expelling the angels from Heaven.
Jane had some angel-like abilities, but wasn't able to show her true power before she died. However, we do know that all Nephilim have extraordinary power, coming from the combination of enokian magic (from their angelic parent) and a human soul (from their human parent). Because of this combination, Nephilim are more powerful than the angels that sire them, but their strength is in proportion to that of their parents. Jack, as the son of an archangel (Lucifer) will presumably be one of the most powerful beings in existence, with all the powers of Lucifer amplified by his human soul.
Castiel's Vision Vs. Dagon's Assumptions
This extreme power explains why everyone was trying to control Kelly during her short pregnancy. The Winchesters wanted to prevent any being with this amount of power from being born, Dagon (Ali Ahn) wanted to harness that power to rule alongside Lucifer and his child, and Castiel wanted to protect the child, believing it could be good.
While Dagon was holding Kelly, she revealed that Kelly would die giving birth to her child. Dagon firmly believed that Jack would be evil, bringing destruction and chaos, and ruling alongside Lucifer. She refuted Kelly's belief that the child loved her and saved her from suicide, claiming that he only saved the vessel that he needed to use to be born. However, when Castiel rescued Kelly from Dagon, he did it with the help of the baby. A golden power flowed from Kelly's hand to Castiel, making his eyes glow gold and giving him the power to blast Dagon out of existence. This also gave Castiel a vision, one of Jack as a force for good, creating a world without pain or suffering, a paradise on Earth.