The Crystal Key is a item that can craft to be used to open the door in Okuyia Cavern, which spawns the Crystal Demon. The Crystal Key can be crafted by Jako in the Demon Slayer Corps using the following items:
- 10 Demon Horns
- 10 Sun Ore
- 1 Demon Collar
- 1 Green Horn
- 500 Yen
- need to be a Slayer