In the Vikk watchtower (M1,20), after defeating a large group of harpies, you can enter the ruins. At the entrance you will find a corpse with clues about the treasure. Jump to the lower level of the ruins and go to the back of the tower where you can find stairs.
Climb to the upper floor. There you must jump to the highest ledge through collapsed architecture parts. Once you reach the top, you will find a chest containing Pyerog rune diagram and New Moon trousers.