Warning: Major SPOILERS ahead for Crisis on Infinite Earths.
Part 3 of "Crisis on Infinite Earths" ended on a massive cliffhanger, making it far bigger than anything that's ever been attempted by any of the five Arrowverse shows. With only a handful of main characters left alive, the heroes of the Arrowverse are undoubtedly facing their darkest hour.
In the third part of the crossover, which aired as an episode of The Flash, Barry (Grant Gustin) found the device that would destroy Earth-1, the antimatter cannon, which was being powered by Earth-90's Barry Allen (John Wesley Shipp). Earth-90's Barry sacrificed himself to save the multiverse, but it wasn't enough. The Anti-Monitor's trump card was Harbinger (Audrey Marie Anderson), who he used to kill the Monitor (LaMonica Garrett).
Due to quick thinking by Pariah (Tom Cavanagh), the seven paragons were spirited away before Earth-1 could be destroyed by the antimatter. One of the seven paragons, Earth-96's Superman (Brandon Routh), was replaced and killed by a magic spell cast by Lex Luthor (Jon Cryer). The Infinite Earths are all gone, leaving all hope in the hands of Supergirl (Melissa Benoist), Batwoman (Ruby Rose), Flash, Martian Manhunter (David Harewood), White Canary (Caity Lotz), Ryan Choi (Osric Chau), Lex Luthor, and presumably Oliver Queen (Stephen Amell), who was in Purgatory with Jim Corrigan (Stephen Lobo). These represent, respectively, the paragons of Hope, Courage, Love, Destiny, Honor, Humanity, and Superman was Truth.
The ending of "Crisis on Infinite Earths - Part 3" also draws several parallels to the MCU's Avengers: Infinity War, which ended with half of the universe being wiped out by Thanos and the Infinity Gauntlet. Only a small number of MCU heroes were left standing. The situation in "Crisis on Infinite Earths" is actually even worse, since the Anti-Monitor destroyed a lot more than just half of one universe. Somehow, eight characters are going to have to work together and pull out a victory over the Anti-Monitor. However, it will be a month before fans will get to see how this happens. The five-part event was split up so that the first three hours aired across three consecutive nights. The last two (Arrow and Legends of Tomorrow) will air together in one night on January 14.
The annual Arrowverse crossovers always air in the same week, and have never been interrupted by a month-long break, so "Crisis on Infinite Earths" is a first for the Arrowverse in more ways than one. Arrowverse producer Marc Guggenheim has confirmed that it wasn't a coincidence that "Crisis" was scheduled this way. Based on where things were left, it seems that the Arrowverse did indeed pick the best -- or perhaps the worst -- place for a cliffhanger, which is sure to fuel enthusiasm and speculation over the event's return.