Spoilers ahead for tonight's episode of DC's Legends of Tomorrow, titled "Amazing Grace."
Tonight on DC's Legends of Tomorrow, the crew of the Waverider found themselves traveling to 1954 to find Elvis Presley, who had come into possession of a guitar that had the long-lost Death Totem affixed to its head -- and along the way, his supernatural backstory tied back to a story told on Constantine's first season on NBC.
This is particularly notable, of course, since John Constantine (Matt Ryan) is a recurring guest in this season of Legends.
On the Constantine episode "The Devil's Vinyl," John found himself in a race to acquire a record that had been imprinted with a song that could control or kill the listener. The story referenced Robert Johnson, a bluesman who, in the real world, reportedly sold his soul to the devil to get his musical skills.
Per the episode summary on the Arrowverse Wiki, "In the 1930s, Willie Cole was recording on an acetate at Moonrise Studios, before the Devil returned to reclaim his soul, killing him. In doing this, its voice was recorded on to the acetate. Marcus Mooney discovered this and had the disc embedded in a wall of the recording studio.
"Around 80 years later, Jasmine Fell uncovered the acetate and took it back to a recording studio to perform a spectral analysis on it. Bernie Reed, despite strict instruction, played it and it burnt off his ears, also forcing him to stab himself. Jasmine, watching Bernie die, took it back to her own place and hid it within her collection. Whilst there, her daughter Julilah found it and began to play it. Before she could be harmed, her parents took it off and wrapped it back up. However, Papa Midnite caught wind of the acetate, sending his men to retrieve it. Rather than returning it back to him, they took it to a club to play it, killing nearly everyone inside. Ambitious, they then took it to a radio station. Though hurting everyone inside the station, they were unable to broadcast the music fully, and were damned to Hell along with the acetate by John Constantine."
Tonight, it was discovered the Johnson's guitar housed the Death Totem and, after he gave it up, that is the guitar that fell to Elvis.
At one point in tonight's episode, Sara (Caity Lotz) even said she had spoken with Constantine about the guitar, and it was he who informed her that she was dealing with the Death Totem. He did not, however, mention the events of "The Devil's Vinyl."
(There is, of course, plenty of opportunity for an Easter egg to happen when he returns next week in "Necromancing the Stone.")
DC's Legends of Tomorrow airs on Monday nights at 8 p.m. ET/PT on The CW.