Fans of both Lucifer and Constantine are wondering just what the infamous Laughing Magician did to earn a favor from The Devil himself. So are the writers and producers of Lucifer, who were as surprised as everyone else by the revelation, which came during the third part of the Arrowverse Crisis On Infinite Earths event. What exactly Constantine did for Lucifer is still a mystery.
One subplot of Crisis on Infinite Earths saw blue-collar con-man/wizard John Constantine recruited to assist in the resurrection of the fallen Green Arrow. Unfortunately, the death of multiple universes was generating cosmic turbulence which made the usual rules of magic unreliable, and this left John unable to work his usual soul-retrieval ritual. Luckily, John knew a magical being who owed him a favor and had the power to open a portal to Purgatory: Lucifer Morningstar of Earth-666, who begrudgingly agreed to help him because "I owe you for Maze." The Maze in question is Mazikeen, a regular character on Lucifer; Maze is a demon who was Hell's most brutal torturer before she left to join Lucifer in his retirement running a Los Angeles nightclub.
Precisely what John Constantine did to help Maze and earn a favor from Lucifer is left a mystery, and it is also a mystery to Lucifer show-runners Joe Henderson and Ildy Modrovich. In an interview with EW, Henderson credited the idea for how Lucifer was brought into Crisis to The Flash writer Lauren Certo, while noting that he had thoughts as to what the favor might be. "I have theories [about Lucifer and Constantine’s past relationship] and we should explore it in the spin-off," Henderson joked as Modrovich laughed. (A Lucifer/Constantine spin-off is not in the works, though one was pitched on Twitter by writer Marc Bernardin.)
This still leaves the question of just what Constantine did to help Maze up in the air, but the idea is not without precedent. John is famous (and infamous) for doing favors for supernatural beings in every faction you can name because, to quote John himself in Hellblazer #60, "there's only one bloke who doesn't play favorites — who does this stuff cos' it's a laugh." In the past, John has undertaken jobs for the Swamp Thing, Dream of the Endless from The Sandman and various demons who needed a mortal magician's aid. The aforementioned Hellblazer #60, for instance, saw John helping hide a pregnant demon from her masters because she had honestly fallen in love with the angel she was meant to be seducing to darkness.
Unfortunately there's no precise precedent in the comics for precisely what the favor to Lucifer might be, as Lucifer and Constantine never encountered one another in the pages of the comics until this year. Amusingly, this occurred in Lucifer #15; a comic that came out one week after the two characters met in the Arrowverse. We can safely presume , however, that it was something impressive if it left Lucifer Morningstar feeling honestly indebted despite his obvious disdain for John Constantine.
On the other hand, it can't have been too huge a favor, since Constantine likely would have cashed in a favor from The Devil Himself to buy back his own damned soul if that were an option. John would also have tried to buy the freedom of Astra, the girl he accidentally sent to Hell. Sadly, it seems the only way fans of both shows and the Arrowverse will ever get an answer is if Matt Ryan does a cameo in Lucifer season 5 or if Tom Ellis shows up to tell the tale on Legends of Tomorrow.