in the 3e NWN campaign video game by bioware the underdark was reserved for lvl 20+ epic characters due to the insane levels of CR the average monster has down there, almost everything down there can kill you in a variety of horrible ways, I loved how my first venture into the underdark in this game I was like "wait I'm only lvl 4 and I'm already in the underdark getting my daily dose of CBT by mr minotaur"
I vaguely remember that there even though its been years since I've played that one there. Just the entrance is protected by a boss with 100 hp more than you and can summon more spiders and they usually have you hurting before the boss spider gets near you
