This mystery can be found in eastern Hamtunscire, just east of the city of Wincestre. You’ll find the cavern entrance as you approach it from the north.
How to Find the Mysterious Mantle Tablet
Once you’re inside the cavern, you’ll find a note stuck to a basket by a knife. Be sure to read it for some context regarding the use of the cavern. After that, proceed through the hole to the southeast, into the main chamber. You’ll find some minor treasure in a chest in the southwestern corner. Once you have the treasure, ignore the surrounding ladders and continue through the cavern to the southeast, following the rock ledge on the left side for the easiest and quickest path.
This will eventually lead to another ledge on your right, accessed from a wooden platform. Follow the ledge to a large branch jutting out of the rock, then jump to the next ledge further south. Follow this over the stalagmites until you reach a chamber with another ledge marked by a poison cloud. You’ll need to take out your torch here so you can disperse the poison clouds with fire, then continue north to the final chamber. This chamber is filled with poison clouds, but you can throw your torches at them to create a safe path to the Mysterious Mantle Tablet. Once you have it, the quest will complete.