X-Men (2013) #21

"EXOGENOUS" Part 4 of 5! As the X-Men race to find the origins of the threat that has emerged from deep space, they discover that there's a larger conspiracy at work...one that may have its roots close to home! What are Agent Brand's ties to the mysterious Providian order? How could Deathbird's secret upset the balance of the cosmos? And what does the third Summers brother - and Rachel Grey's uncle Vulcan - have to do with it all?