It’s funny. The May/June/July issues of every single X-book are lead-ins to Onslaught, and yet the event itself did far more to the non-Xverse than to the mutants themselves–“killing” major characters like Captain America and Iron Man.
Onslaught himself, who sought mutant domination, probably would have liked it that way!
These issues have Cable and Domino looking into the coming of Onslaught. Cable figures out that Onslaught is Professor X…
…But Post beats him nearly to death, so he can’t warn the others.
The next issues are covered in the post on the Onslaught Event.