Tattered Note

Tattered Note is a piece of parchment at the base of a cave in blocking the mouth of the Mysterious Den in the Red Stone Run, on the eastern side of the Timeless Isle.

This suggests to find one of the Timeless Legends on the isle, and use the provided bonus ability on the Cave-In. Should players somehow find themselves locked inside the mysterious den, there is a Rock-breaking Hammer on the inside that can be used to break the cave-in as well. Either method of breaking the cave-in will immediately summon Spelurk, a rare mistlurker.