I wonder if Onslaught has something to do with The Trial of Magneto coming up? The way the last page of Way of X is composed def makes it seem like Onslaught is going to do something during the gala. We’ll see.
The solicitation for issue #4 too. It reads "Kill No Man!" and "Nightcrawler must act fast to avoid catastrophe as the laws of Krakoa (and physics) are tested to their limits." So that could also fit with Onslaught-related mischief being involved in whatever Magneto does (or appears to do). David and Kurt could be the ones who force Xavier to hold a trial, to save Erik from getting thrown in the pit.