Forest of the Mother Trees and the Hadozee tree villages
by Marc SaindonPerhaps placed just east of Tangor Bay, this realm of giant trees allows to place entire "dungeons" into one big plant, even an entire ecosystem, with giant version of known garden pests (giant spiders and the like). Since Araneas and (delicious) Phanatons already pop up all over the place in Mystara, why not just place Hadozee (Monkeyfolk) dwelling in these trees? The biggest Mother Trees (not to be confused with the elven Trees of Life) sometimes host more than one settlement, and skirmishes might occur between them.
The Hadozee don't really have a backstory, aside that an Archfey living in the Mother Tree Forest heard about them from a planar traveler and, intrigued, used potent fae magic to artificially evolve his own from the local apes. Hence, the Hadozee will tend to Rogues, Rangers, and Druids, while Clerics and Paladins are unheard of (something intentionally cultivated by the Fae). They don't have the nickname "deck apes", as the Tangor Empire has only recently built its navy and has not yet recruited from these forest-dwelling folk, usually left to their own device by imperial officials (who couldn't be bothered to climb up these trees).
(Image from: https://www.deviantart.com/sonofamortician/art/Ancient-Baobab-92672759)