This mystery can be found in the northwest of Suthsexe, just south of Chertsey Abbey Ruins, AC Valhalla.
Where to Find the Mad Monk
As you approach the tomb you’ll hear the voice of a Mad Monk. When you reach the entrance, you’ll notice that it’s blocked by a movable barricade. You won’t be able to move the barricade inward until you destroy a crate on the other side of it.
How to Enter the Tomb
To destroy the crate, go to the northeastern side of the tomb and shoot the crate through the hole in the stone. You can now go back to the southwestern side and move the barricade inward. Once inside, loot everything you can find and then leave the tomb. As you do, the Mad Monk will try to ambush you. You’ll need to defeat him in combat to complete the mystery.
The Mad Monk wields a two-handed hammer which can inflict a great deal of damage if you’re hit by it. The main thing you’ll need to look out for is the red rune aura attack, which the Mad Monk uses fairly frequently. Most of his attacks are slow and predictable, so you shouldn’t have any trouble defeating him.