Burning Over the Undersky
The lizardfolk call it Kir Sen So, the Enfolding Green. They had a powerful civilization once upon a time, now only revealed through foreboding stone ruins dotted throughout the jungle. There are remnants of the old Scaled Empire in the remains of the culture that survive: lizardfolk have a collection of their own gods and ceremonies that have been maintained, some of which are shared by the humans and orcs that live there as well.
Trees grow closely together and the canopy is densely woven together, strongly enough to bear the weight of the yuan-ti civilization. The yuan-ti have no truck with the ground-dwellers and a bitter resentment towards the lizardfolk in any case.
Heavy rain is frequent, with sunlight mottling through the overgrowth at irregular intervals. Even when not raining, the jungle is in deep shadow throughout.
Lizardfolk are the most populous race here; some still live in the vine-choked ruins of their old empire, but most live in riverbed communities. Since the old days, they have diverged in certain regions: some are eight-foot tall monstrosities with armored skin, powerful tails, and savage jaws, that bear enormous clubs and stalk prey relentlessly; others are small, chameleon-skinned skirmishers that use poison-tipped javelins; others still are the standard medium-sized lizardfolk of popular lore. Lizardfolk rarely war against one another, although they have been used as armies and pawns by green dragons in the not-too-distant past.
Orcs survive in caves and amongst the roots of trees in tribal communities. They trade with the other denizens and war with the yuan-ti for access to the sky.
Humans make huts and clear areas of the jungle near the rivers, where they float canoes and flatboats to trade with other communities. Some have established regular routes all the way to the plains and the swamp; once a generation or so some intrepid group will attempt to ford upstream to trade with the dwarves.
Yuan-ti live in the treetops hundreds of feet above the ground, except when they can establish a nest in one of the various ruins before the lizardfolk discover it. They trade, but only under certain circumstances that no one else has been able to determine. They have their own gods and do not allow others to see their ceremonies.
The jungle is deadly, with a plethora of creatures to be wary of: jaguars, peccaries, crocodiles, snakes; more exotic dangers include gricks, bullywugs, trolls, even a few green dragons.