Marvel Comics' FCBD 2020 X-Men #1 showcases some teases for the future of mutantkind as 'X of Swords' approaches, but it also holds a solicited secondary tale that foreshadows a major story titled Dark Ages coming from writer Tom Taylor and artists Iban Coello and Brian Reber, which hints at something big coming to the rest of the Marvel Universe later this year.
Along with a double-page spread announcing Dark Ages for release in fall of this year, there's little to go on about the story – however a handful of preview pages by Coelle and Reber set the stage. In the preview, Tony Stark and Pepper Potts deal with a mysterious phenomenon that attacks New York City which seems to cause an earthquake, eventually leading to the disruption of all electronic systems in the city – including Tony's Iron Man armor. As Tony tries to reach Pepper while she is stuck in an elevator, his armor fails and he's sucked into a jet engine, falling thousands of feet to the Earth, losing his leg and possibly even his life.
In one of his last acts before his crash and possible death, Tony calls Captain America, saying simply "Avengers Assemble" before being cut off by the loss of electronics. Though there's no indication of what's behind the blackout, a mysterious green energy is seen collecting in the sky over Manhattan leading up to the loss of electronics. Just before he loses consciousness as Pepper stands over his body, Tony says "Dark... It's all gone dark... Forever."
No effects of the blackout outside of New York are shown in the pages, however the teaser calls Dark Ages "a saga of the Marvel Universe," indicating that the problem will extend well past New York.
Marvel hasn't specified whether Dark Ages will be a one-shot, limited series, ongoing series, or crossover. It's scheduled to kick off some time this fall.