Where demons come from is no secret. They spawn out from the "Mother", endlessly. 

If the "Mother" was destroyed (something I am not entirely sure is possible) then the remaining demons would still be legion, and in retaliation the Masters would unleash them.

Still, the TRUE origin of demons is shrouded in the mists of the past. Even the "Originals" seem hazy about much of what happened, or else are simply unwilling to access those memories. 

I personally believe they must have come from "The Brother", somehow. 

All of the Demon Masters are "Originals", of course. There are a few Demon "nobility" that are also "Originals", or else claim to be. The Angels have bound and sealed a handful of "Originals" on the surface as well.

I must simply hope that no more "Originals" can come into being. Demons with no limit on their energy storage... it is hard to maintain such a balance.

-Hidden writings of the Demon Master Dzu