The tetrahedron 2 is an item used in Ritual of the Mahjarrat. It is found by searching rubble on the west side of Kethsi.
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.
Where the tetrahedron can be found.