Starblazer

Hellstar is the slash ship between John Constantine and Lucifer Morningstar from the Arrowverse fandom.

Crisis on Infinite Earths
Constantine, Diggle and Mia go to Earth-666, Diggle asking why they are there. Constantine explains that due to the multiverse ending, magic is running out and he can't get into purgatory to save Oliver. But he does know someone who can, and knocks on a door in an alley. The door opens to reveal Lucifer, chatting up some girls, becoming stunned when he see's Constantine. Constantine says that it's time, and Lucifer turns away the girls to talk to Constantine. Lucifer says that he would say that it's a pleasure to see him but it's not, then notices Mia. Constantine interrupts their talking to tell him that they need to help Oliver get his soul back. Lucifer says he'll do it, so that they'll be even for Maze. He pulls out a card and hands it to Constantine, explaining how it works. He says that he can consider them even, mispronouncing Constantine. Constantine corrects him, but Lucifer says that he doesn't care and walks back into the club.

Crisis on Infinite Earths

 * John calls Lucifer, "Lucy".
 * Lucifer looks John over as he walks back into the club.

Fanon
Although there were fans who shipped them before Crisis, and had wanted them to meet at some point, the ship took off following Lucifer's cameo in the Crisis crossover. Most speculated that the way that Lucifer and Constantine knew each other was because they hooked up. Or, had hooked up when Constantine was helping Lucifer with Maze.

On AO3, Hellstar is the fifth most written ship for Constantine. It is also the sixth most written ship in the Constantine (TV) tag.

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Trivia

 * Lucifer mispronuncing Constantine, is a reference to the comics where it's actually pronounce "Constan-tine", instead of "Constan-teen".