Excalibur: Weird War III (Marvel Graphic Novel #66) (1990)
Excalibur finds itself in an altered Earth where the alternate, Nazi-controlled reality they recently visited has merged with their own. The Third Reich in control of this new reality is experimenting on mutants. With Hitler a cripple under the care of the Red Skull, Charles Xavier is the de facto head of the Reich. Excalibur must keep themselves alive in this horrifying new world and find a way to separate the two realities to restore their home.