Batman and Superman's journey to find Boy Thunder has taken them across the multiverse to the universe of Kingdom Come. Their journey in Batman/Superman: World's Finest #20 (by Mark Waid, Dan Mora, Tamra Bonvillain, and Steve Wands) was not straightforward, however. Their trip to this reality shunted them forward in time, making them bear witness to this universe's ultimate fate: a great tragedy in which many heroes were killed by a nuclear bomb. While both men were rattled by this, Superman's perspective afterward was both surprising and concerning.
Source：https://www.cbr.com/superman-kingdom-come-failure-dc/
