A tetrahedron 1 an item used in Ritual of the Mahjarrat. It may be found in a secret compartment in a wall design on the 1st floor of the accessible part of Kethsi. This is just north of the fairy ring.
To open the compartment, Bob's collar must be used on it, and then the pattern on the collar must be matched to that on the compartment via an interface. A Note to you, Note to robert, statue arm and Robert's necklace will also be found by opening the compartment.
The tetrahedron should be taken to the underground area, along with the other three tetrahedrons, and placed in an indentation in the wall. This will give a letter which, when combined with the letters with the other tetrahedrons, will form a code that corresponds to a location. The code's location can then be found using the strange device.