UnderMine

When entering a new room on any of the floors in the game, a pile of bones with a skull can spawn somewhere in the room. Walking up to the skeleton and interacting with it (Pressing on the controller) has a chance of dropping an item or nothing at all. The skeleton can drop bombs, keys, gold, and Thorium after interacting with it. The player or enemies in the room can destroy the pile of bones before you can interact with it. So be careful!

When interacting with the skeleton, it has a chance of coming back to life and attacking the player. If the skeleton comes back to life, the player will hear the doors in the room slam shut (if they are open) and the skeleton will stand up. The Player has a couple of seconds to dodge out of the way before it attacks.

After successfully interacting with 100 skeletons, the achievement will unlock. The achievement can still unlock if the player gets nothing from the skeletons. You only have to interact with them to meet the requirements and unlock the achievement.