Supernatural and Lucifer have mutually dropped Easter eggs and references to connect to each other over the years. From the mind of Eric Kripke, Supernatural first aired in 2005 and has continued for an impressive 15 seasons, with the final, fifteenth run currently delayed due to the pandemic. Jared Padalecki and Jensen Ackles star as Sam and Dean, the Winchester brothers, who take it upon themselves to hunt the monsters and demons dotted throughout the U.S. Though they start with low-level enemies, the Winchesters soon come up against Lucifer himself, played expertly (is that a compliment?) by Mark Pellegrino. Transforming into arguably Supernatural's best villain, Lucifer has been a recurring figure on the series for almost a decade.
Always a popular source of inspiration, Supernatural isn't the Devil's only representation on network TV. In 2016, Fox began airing Lucifer, based on the hellish Neil Gaiman creation for DC comics. The story finds a bored Devil (played by Tom Ellis) relocating from his fiery depths to the sunny climes of Los Angeles, where he runs a club and helps local law enforcement solve crimes. Aside from the presence of Lucifer himself, the two shows are thematically similar and orbit similar worlds. The Devil's earthly presence in Lucifer attracts all manner of celestial and biblical arrivals, some of whom have names that Supernatural fans will find familiar.
The best reference between Supernatural and Lucifer comes in Supernatural's eleventh season. The appropriately titled "The Devil In The Details" sees Pellegrino's Lucifer make yet another attempt at freedom, but Sam Winchester is thankfully on hand to stop him. As Sam speaks to the caged Lucifer, he asks what the Devil would do if he breaks out, to which the villain replies, "I move to L.A., solve crimes." This is a neat reference to Supernatural Lucifer's Tom Ellis-shaped counterpart, alluding to this alternate version of the character. However, Lucifer has struck back with its own nods towards Supernatural.
In Lucifer's fourth season - after being cancelled by Fox and subsequently revived on Netflix - a nephilim is conceived between Linda and Amenadiel. Although the baby is eventually named Charlie, one of the other suggestions is Jack, which could certainly be a reference to Supernatural's own half-angel, Jack Kline, the son of Lucifer. In the same season, Amenadiel and Ramiel discuss their fellow angel Cassiel and his awful singing. Cassiel was the inspiration for Castiel on Supernatural, and indeed, season 9's "Heaven Can Wait" features Misha Collins' character warbling badly trying to get a baby to sleep.
The references between Supernatural and Lucifer are perhaps partly due to the shows being connected behind the scenes. Both film in British Colombia, both are produced by Warner Bros., both have shared guest stars (Aimee Garcia even plays similar characters), and Richard Speight Jr., better known as Supernatural's Gabriel, has directed episodes of both. With such close ties, it's perhaps unsurprising that Supernatural and Lucifer would bring their working relationship on-screen.
Unfortunately, the Easter eggs don't quite amount to evidence that Supernatural and Lucifer could potentially exist in the same universe, as much as a crossover would delight fans of both shows. Supernatural's parallel universe setup could, theoretically, make this possible since each of God's worlds has its own set of angels, but the same logic could equally apply to any TV series. However, Lucifer's comment about moving to L.A. and solving crimes in Supernatural might reveal that Lucifer exists either as a TV series or as the original DC comics in Supernatural's world, and that the Devil is highly amused at what a certain Mr. Gaiman thinks he would get up to in the mortal realm.
Supernatural season 15 and Lucifer season 5 are both halted due to the pandemic.