Savage Tide is a veritable
book tho.
But those two things are really helpful, info-wise, and sort of line up for what I'm going for.
I'm running a Jungle Exploration hexcrawl game, an expedition into a newly discovered continent. The tone and feel is pulpy Jungle action--dark ziggurats, sacrifices to volcano gods, evil snakemen, curses and shrunken heads and apes and carnivorous plants and pirates and so on. A giant two-headed demonic monkey makes for a great lurking source.
With this setting, I've went with the 4e idea that Demons are the result of abyssal corruption on anything it touches, and one of the easiest things to corrupt is the natural world (and elemental forces, which are in the Abyss's back yard). The prior advanced civilization fell when demons ran amok*, corrupting everything, and then the ensuing efforts of fey and nature spirits to stop it, lock it down and ward it away. It left Demogorgon with a foothold in the plane, tied to the land (albeit currently locked).
So for me, Demo isn't just
madness but tearing creation apart under feral brutality. Reducing all of existence to sharks in a feeding frenzy, running on the basest and harshest instincts. Rabies given form, basically.
*What comes to mind on how this happened, and Demo was involved, is that the prior civilization were heavy into astrology and prophecy. And somehow Demo tricked them (by way of manipulating the stars?) into worshiping him, in enacting rituals that corrupted them, devolved them, and spread demonic contagion. He also slew the local volcano goddess by tearing out her heart, an artifact that can be found and dropped into a volcano to rebirth her.
I also have this idea of Demogorgon's two heads originally being one that was split in half, but not really sure how to work that in. It would also mean his two-headed nature is "recent", in terms of time.