Captain America #174 Jun 1974
The original X-Men appear in "It's Always Darkest...!"
Cap and Falcon are disguised as possible employees of the Secret Empire to gain entry into their secret base to spy on what they are planning.
They are found out and almost executed by laser beams. They are able to escape down an exhaust vent and then find a way to let in
Professor X, Cyclops and Marvel Girl who are looking for lost X-Men and some other X-Men's villains who have mysteriously disappeared.
They locate the missing mutants, Beast, Iceman, Angel, Lorna Dane, Havok, Mastermind, Blob, Unus and Mesmero
who have been attached to a turntable that will use their mental patterns to project a force field.
The mutants are set free, but then the Secret Empire uses the Atomic Annihilator on the heroes
and appears to kill them all!
Script by Steve Englehart,
pencils by Sal Buscema,
inks by Vince Colletta