Warning: Spoilers for Supergirl season 5, episode 8, "The Wrath of Rama Khan".
The final episode of Supergirl heading into Crisis on Infinite Earths confirmed that the Lex Luthor of Earth-38 had been resurrected by the Monitor to save the multiverse. While preview pictures of the Crisis event depicted actor Jon Cryer as Lex Luthor, there was no indication that it was the same version of Lex Luthor he had played previously.
Luthor's return for Crisis had been theorized by fans following Supergirl's season 4 finale. Luthor successfully faked his death while battling Supergirl, successfully teleporting away at the last moment before his power armor exploded. Unfortunately for Lex, his sister Lena guessed what he was up to and was waiting for him in his secret hideout with a loaded gun. However, this didn't seem to be the end for Lex Luthor, as the final scene of the season 4 finale showed the Monitor inspecting his dead body as his hand began to glow.
Supergirl season 5's midseason finale, "The Wrath of Rama Khan", ended with a scene that confirmed Luthor's resurrection. Lex was seen on a platform in space; the abode of the Monitor first seen during the The Monitor ignored this and stoically told Lex that the Monitor brought him back to life because "the universe will need a mind like yours" and that he could now "become the hero that you have longed to be." Lex responded that he had always been a hero, but that he was happy to help the Monitor if it meant saving all reality.
Lex Luthor playing an important role in Crisis in no surprise, as multiple Luthors played major roles in the original Crisis on Infinite Earths comic book miniseries. The Lex Luthor of Earth-1 became leader of an army of supervillains who originally plotted to conquer the surviving worlds only to be recruited for a mission to save all reality from the Anti-Monitor. The Lex Luthor of Earth-3, in an ironic parallel of Superman's origins, sent his infant son Alexander Luthor Jr. to Earth-1 in a dimension-traveling rocket ship. The journey accelerated the baby's aging and gave him the ability to manipulate matter and anti-matter, making him into an ideal hero to lead the charge against the Anti-Monitor.
It remains to be seen what role Lex Luthor will play in the television Crisis storyline. Historically, Lex has played it straight when forced to work alongside superheroes just as much as he's tried to double-cross them and try to win some kind of advantage while their backs were turned. Whatever the case this time, Lex's return is sure to have a major effect on the cast of Supergirl; particularly his sister, Lena.