Bone Key
The key to Lighthouse Dungeon 5F.
Etc. Items
Sell Price to Secret Vendors: 0 
Gold
Sell Price at Black Trader: 5K-30K Gold
How to Obtain:
Trade 
Pastal 
Piece of Bone (1) (Chance)
Used:
Access to Angel's Gate (Lighthouse Dungeon 5F), where 
DEVILANG is located.
Related Items: Captain's Cabin Key
Hallway Key
Rusty Key
Storage Key
Warehouse Key
Notes:
According to RedShana's record, 324 bones had been 310 ashs and 14 bone keys. Rate is around 4.3%.
A piece of bone = 250 Gold
A ash = 150 Gold
Therefore:
The price of key≈(324*250-310*150)/14 = 2464 Gold
But it is very troublesome to make them, so their price is increased.
KimSuhanmu recorded 1665 bones and received 54 keys (3.24% ≈ 3-4%)