Marble Key
From Drunkapedia
A key that is carved using Lockpick skill with an Intricately Craved Tool from a Twisted Dark Key dropped by many monsters that spawn within the Valley of Death. It is used to unlock a Reinforced Oaken Chest which is full of Sturdy Iron type loot and randomly spawns in the Valley of Death. In all the quality of the loot is not worth the hassle of using these keys and they have no other use or value to an NPC.