UnderMine

This a RNG achievement. From time to time, at the end of a level, instead of having the option to drop into the next floor, you'd notice a purplish picture that looks like a set of teeth gnarling at you. Drop down into this area to meet Sho'guul. On offer will be two random effects that you can select. Each effect will give you a variety of curses. You must choose either one to unlock the achievement.

Picture for reference.
https://www.trueachievements.com/gamescreenshot.aspx?gsid=38...

PS: It's possible to leave the lair without choosing an item, and miss out on the achievement.