Marjorie Liu's Astonishing X-Men has stood apart from the freight train that is Brian Michael Bendis taking the driver's seat in big-time X-land. David Lapham's Age of Apocalypse is set in that alternate reality, and Greg Pak's X-Treme X-Men has Dazzler leading a team of dimension-hopping heroes to try and put a stop to evil versions of Charles Xavier. Since Bendis' X-business has little to do with any of these books – we'd doubt he even reads them – that leaves these three fringe X-books free to do fun things, like crossing over with each other.
X-Termination is a new event that will string these books together, picking up on elements from Rick Remender's Uncanny X-Force – namely, that Nightcrawler from the Age of Apocalypse universe is in our world, and thinks very little of it. In fact, he betrayed Wolverine and the X-Force team to head off with Mystique and the Brotherhood. Now, he's looking to head back to his true reality, and he's willing to sacrifice ours to do it.
Here are preview pages for X-Termination Alpha #1, and you can read the authors talking about it over at Marvel.com.