Arrow has been teasing Lyla Michaels' "Crisis on Infinite Earths" character as far back as season 1 of The CW series. It was confirmed in early October through a new image for the upcoming crossover that Lyla will suit up as Harbinger for "Crisis on Infinite Earths". It hasn't been confirmed yet if Earth-1's Lyla will don the costume, or if a Lyla from a different timeline will take on the role of the comic book character.
In DC Comics, Lyla Michaels is Harbinger, a character who plays an important role in the 1980s crossover miniseries, Crisis on Infinite Earths. She was a young orphan lost at sea on Earth-1 who was rescued by the Monitor. After being taken under his wing, Lyla was granted powers and a mission from the Monitor. She was tasked with traveling through the multiverse and gathering the heroes and villains needed to help the Monitor stop his nemesis, the Anti-Monitor. She was eventually possessed and corrupted, which led to Harbinger murdering the Monitor.
Arrow executive producer Marc Guggenheim has confirmed that Lyla Michaels' debut as Harbinger is something that's been teased since the show's first season (via ComicBook). Lyla Michaels was introduced in episode 19 of Arrow season 1 as the ex-wife of Diggle (David Ramsey) and an agent of A.R.G.U.S. In just her second appearance on the show -- which happened in season 1, episode 20 -- the name "Harbinger" was dropped for the first time. While on the radio with other A.R.G.U.S. agents, Lyla was referred to with the codename, "Harbinger". She's used the same codename ever since.
At the time the name was first mentioned, it was thought of as just an Easter egg that would most likely never lead to anything more, considering the differences between Arrow's Lyla and the comic book character. Aside from their names, the two characters have nothing in common and it was difficult to imagine Lyla ever becoming anything close to the costumed superhero who travels between multiple universes in the comic books.
Fast-forward seven years later, and the leap from A.R.G.U.S. agent to the Monitor's cosmic assistant doesn't seem so far-fetched. Many characters in the Arrowverse have come far in recent years, Lyla included. Lyla, who is now the head of A.R.G.U.S., has become akin to Marvel's Nick Fury, a character with vast knowledge and deep connections to important people from all over the world. So in a way, it makes sense that Arrow's Lyla could be given a task similar to her job in the comics. Either way, how Arrow has transformed Lyla is just one more sign of how "Crisis on Infinite Earths" is paying off storylines that have been building for years.