Crystal Golem Cave
The entrance is a trap door surrounded by movable blocks just south of Barbabas' house.
The crystals in the cave heal the Crystal Golems and the golems are immune to regular physical damage. Sneak past the golems or lure them away from the crystals and hit them with magic or fire arrows or the Golem Slayer sword.
The end of the cave has a room with coffins and a chest. One of the runes to open the Underground City is here, as well as part 1 of the Treasure Map.
Closing the door to the room resets all the golems and they will walk back to their original positions.