With the amount of xenos races in the galaxy, especially explored more in 30k, I'd say it could be anyone's (I wouldn't cry foul if you suggested it was a Kinebrach weapon), but it being a Necron weapon is entirely feasible. It does sound a lot like a C'tan phase weapon.
That, or it's a weapon made entirely our of shitty deus ex machina; it seems to be the Alpha Legion's primary resource.
Fair point about the Kinebrach. Do we know that the Kinebrach pre-date the Eldar (since the Pale Spear is supposed to pre-date the Eldar) and do we know much about the weapons they use? I can't find much apart from the anathemae which was poison based