Arrow: Here is How "Purgatory" Sets Up for "Crisis on Infinite Earths"

Tonight's episode of Arrow served as the show's mid-season finale of sorts, but it also marked the final episode of all of the Arrowverse shows before the arrival of "Crisis on Infinite Earths" on Sunday. That placement in the lineup made it almost certain that tonight's "Purgatory" would see the arrival of that "Crisis" and the episode did not disappoint. Arrow brought "Crisis" into its most somber and terrifying focus yet -- and here's how it went down.

Spoilers for tonight's episode of Arrow, "Purgatory," below.

The episode saw Team Arrow -- both current and future -- on Lian Yu where they were to build a weapon that would save the heroes who could stop Crisis. However, there's a bit more to it than that. Once the weapon is assembled, they determine that it's coded to someone's DNA and that would be the only person who can operate it -- and it's not Oliver. It's Lyla and once she turns it on, she suddenly announces that she needs to go, and she disappears through a portal.

The team begins to leave the island and as they do, Oliver takes the opportunity to say goodbye, in a sense, to those he cares about as he fully expects to die in Crisis. He asks Oliver to track down Felicity and have her reconnect William and Mia so they can grow up together. He says goodbye to Roy. He has a touching moment with William, too, before going to more or less say goodbye to his dead on the island. It's there that he has his moment with Mia, the pair coming to an understanding of one another. It's then, when they're ready to leave that everything begins. The skies turn red, just as we saw in The Flash earlier this evening and then Lyla returns. But she's not Lyla this time. No, she's now Harbinger -- and she confirms the Crisis has begun.

"Lyla Michaels as Harbinger is a key component of the original Crisis on Infinite Earths that we were determined to honor in our crossover," executive producer Marc Guggenheim said in a statement earlier this year. "What makes this particularly satisfying for us is that we introduced Lyla Michaels and her Harbinger codename all the way back in Arrow Season 1."

In the comics, Crisis on Infinite Earths centered on a battle between the combined superheroes (and even some villains) of the DC multiverse and an immortal, cosmic threat known as the Anti-Monitor. Like The Monitor, the Anti-Monitor will be played by LaMonica Garrett in the Arrowverse. As the Anti-Monitor destroys realities, he replaces their positive matter energy with antimatter, growing his own power and sphere of influence. He was eventually stopped by the sacrifices of several heroes, including The Flash and Supergirl, as well as the merging of multiple universes to save reality by becoming a single, unified timeline. Fans have long wondered whether the events of "Crisis on Infinite Earths" might bring Supergirl and even Black Lightning to Earth-1, where the rest of the series take place. The crossover will also feature guest appearances by Tom Welling as Clark Kent, Erica Durance as Lois Lane, John Wesley Shipp as the Flash of Earth-90, Johnathon Schaech as Jonah Hex, Kevin Conroy as Bruce Wayne, and Ashley Scott as The Huntress.

"Crisis on Infinite Earths" kicks off on Sunday, December 8 on Supergirl, runs through a Monday episode of Batwoman and that Tuesday's episode of The Flash. That will be the midseason cliffhanger, as the shows go on hiatus for the holidays and return on January 14 to finish out the event with the midseason premiere of Arrow and a "special episode" of DC's Legends of Tomorrow, which launches as a midseason series this year and so will not have an episode on the air before the Crisis.