One of the things that’s as constant in the history of Dungeons & Dragons as the official rules are each group’s house rules. In the group I’m in, the Wyrmfang Chronicles, we have a few of our own. Here’s a couple of ours plus a few that I’ve seen elsewhere.
The Underdark is a dark and dismal place, full of creatures good and evil, though mostly evil. Mindflayers and drow are among the worst of the worst, but by no means all that dwells therein.