The undergrowth is the floor of the Hushwoods. No sunlight can reach the undergrowth because the canopies if the Hushwoods are so thick. Double that with the mist coming from the South Altric Ocean and you got one dark, gloomy, cold, place. After the Iron Giant War, the influx of fey folk and forest spirits caused the trees to grow exponentially, causing the canopies to block out the sun from the undergrowth. As a result, fungal creatures began breeding like crazy, monsters would call the undergrowth their home, and travel under the canopies became very deadly.
For decades, no human or nonhuman willingly ventures into the undergrowth.
There are several ruins of castles, towers, and cities that were part of the Del Maleth Territory, but have been abandoned once the Undergrowth was created.
living in the Undergrowth are small villages of Myconids who are relatively harmless to the people of Hushwoods. But there are also several Blights (and maybe Gulthias trees) on the Undergrowth, too.
There is also a contingent of drow that have made the Undergrowth their home.