This mystery can be found in northern Snotinghamscire, north and slightly east of Hemthorpe, or directly west of Jorvik. To gain entrance to the cavern, approach it from the south.
How to Find the Mysterious Whetstone Tablet
Once you’re inside the Deoraby Spar Cavern, look northwest to find a ledge above you. The base is covered in stalagmites, but you can climb these with some persistence. Once you’re at the top, go through the hole facing northwest to enter a new chamber. In this chamber you’ll find some loot including an Opal and a Deoraby Spar Cavern Door Key. Be sure to loot the key from the skeleton, then use it to open the door nearby to the northeast. You’ll also find an inscription on the stone to the left of the door, the contents of which indicate something of what lies ahead.
Once you’ve unlocked the door, turn immediately to the west and jump over the rocks here, leading to a minor treasure chest. Loot the chest and then turn east, jumping over the gap in the ground to find a Roman Secrets note on the other side, and 4 Titanium next to it. Now climb north up the blue rocks and follow the ledge until you reach the top of the statue of a person with an object in their hand. Jump to the object and then onto the pillar to the west. You can now jump over the stalagmites to find another minor treasure chest to the northwest.
From the second treasure chest, turn west and climb up the fallen statue. Beyond it you’ll find a room containing the Mysterious Whetstone Tablet, alongside a Whetstone’s Inscription note. You can then exit the cavern by destroying the link on the door to the southwest.