Balance, order, disorder... chaos.
Perhaps the source (good) is the inverse of the aliens core 'source' of power (evil).
Maybe the source was sealed to create balance, to prevent the oposing powers from attempting to destroy it and have full reign (though usually destroying the balance causes everything to cave in, the two sides seldom realise this).
The beast was the seal, we severed the seal, causing mass fluctuations, undoing the balance, causing loss of the oposite (dying aliens). They can now see where the source is again, perhaps the seal kept it from being rediscovered?
So now they know where their arch-nemises is, they seek to challenge it, and we (using the source, notum, the 'good') represent the source itself just as the aliens are the representitives of their source of power.
Now another climactic battle rages, beyond the beast, the armegedon. Good and Evil, in their most dominant forms, meet to face off yet again. Which means we are helping to keep the balance here, since we screwed it up anyway ;p
----------------------------------------
Just my most recent thought on the matter, this is getting me intrigued now, hehe.