Joker said:
About a millionth of the Edition War's size. Or the D&D Race War. Or the RPG-System War. or the War between Old and New WoD.
Is it on such a scale that not even the gods can comprehend it?
With them being more or less omniscient, I doubt it. But surely it's way beyond everything any non-deity could ever hope to comprehend.
Will it ever tip in favor of one of the other side?
Who knows? It has lasted Aeons (it was there before time), it might last until the very end (but not much long after - according to current info, devils draw their energy from the souls of the damned. Once the flow of damned souls ends, due to the Apocalypse, the devils' power will run out sooner or later. They won't be able to promote other devils, they won't be able to create lemures out of the damned, so they will run out of troops. So it seems that after the last mortal on the last material world has died, the devils will lose the fight, the demons will overrun first them and then everything else).
But of course it's quite possible that the information we have is wrong...
Which major players have a vested interest in seeing the Blood War continue?
I think most have, because if any side should prove victorious, it would probably set out to conquer or destroy the rest. Plus, the 'loths profit from the Blood War, so they don't want it ever to stop.
What are some good sources for information about the Blood War?
Older edition material; and both Fiendish Codexes have some tidbits about the whole affair.
Can it ever be influenced in any major way?
If you consider that there are millions of combatants (if not more) on either side, you'd have to do something beyond Epic to influence it.