Repeatedly it is said that Magneto lost half of his powers while trying to destroy some Apocalypse starship. Do you know the name of the comic, or is it just a fact mentioned - but never depicted - in the story?
This happened in X-Men Chronicles 1: "Origins" (1995)
In the Age of Apocalypse, Ship was nearly destroyed by Magneto when it endangered the lives of his X-Men. This was after they successfully thwarted the plans of En Sabah Nur's Horsemen to take control of the United States nuclear arsenal. The battle between Ship and Magneto left the X-Men leader drained and unable to ever fully restore his magnetic powers.
From the referenced comic:
Magneto overloads his powers in an attempt to stop Gideon and avert the launch of nuclear missiles.
Later, while still weakened from his previous exertion, Magneto damages Apocalypse's ship.
The Wikipedia description seems a bit confusing, since Magneto didn't entirely destroy the ship, and calling it a "battle between Ship and Magneto" seems a bit inaccurate, but it does occur after the Horsemen try to take control of the nuclear arsenal, so....