Very interesting. Some kind of alien weapon (perhaps a beam of some kind bounced off of an old satellite?) instead of the nuclear missiles would make for a pretty impressive mcguffin. It would also explain the BoS and Enclave presence quite handily, and their mutual urgency in taking on The Empire.

It would be rather interesting if such a dangerous foe became the catalyst for the two groups being forced to work together, at least temporarily. (A la the BoS and NCR in the Mojave in the Battle for Hoover Dam against their mutual foe.)