Collects X-Men (2019) #16-20. One era ends and a new one begins as Jonathan Hickman's acclaimed run enters the Reign of X! In the wake of the Earth-shattering events of X OF SWORDS, the Captain Commander of Krakoa makes a fateful decision that will affect all the island-nation's residents! And when the Shi'ar Empire asks the X-Men for help, Cyclops, Storm and Marvel Girl answer the call! Elsewhere, the door to the Vault swings open at last. But time moves differently inside the Vault - and for the beings who emerge, it's been a very, very long time since they entered! And Krakoa's future is at stake when one of mutantkind's greatest threats re-emerges: Nimrod is online! Then, it's time for the Hellfire Gala - and a changing of the guard, as Krakoa's very first team of elected X-Men debuts!