Penciler: Randall Green
In the aftermath of Onslaught, a de-powered Professor X is imprisoned in an abandoned Hulkbuster base without hope of freedom. Will Charles Xavier find an ally in prisoner M-9, a young girl with incredible mutant abilities?
You could just get the above three issues and the story would work, but if you wanted the full Onslaught storyline Marvel did an Omnibus for that too.
X-Men/Avengers: Onslaught (Hardcover)
Published: August 19, 2015 (1996-97 storyline)
Onslaught is here! The fury of Magneto plus the psionic might of Charles Xavier equals very bad news for the Marvel Universe! Now, discover how this mental monster came to be – and the full extent of the havoc he wreaked on the X-Men, the Avengers and pretty much everybody else! As the truth slowly dawns about the nature of the evil they face, how much are Marvel’s greatest heroes prepared to sacrifice to save the world? Collecting CABLE (1993) #32-36; UNCANNY X-MEN (1981) #333-337; X-FORCE (1991) #5