Colossal Cave

When you first enter the cave, you'll eventually come to a big room full of debris, water flowing from one side and a big sign with the word "XYZZY" on the opposite side. The narrator notes that the water is bringing debris down to this part of the cave. Look where all the debris is and you'll see the Black Rod, which is kinda like a magic wand. Pick it up.

Proceed onwards and you'll eventually trigger a scene where a Dwarf will appear and throw an axe at you before vanishing. Turn left and go west until you see an unfinished or broken bridge, and use the Black Rod on it to fix it and pop the achievement.