The item is used to improve Martyr’s blade and unlock the first skill. We will tell you where to find it.
In the Abandoned Chambers where you will unlock the Martyr’s blade, go deeper into the location. Look for a hole in the wall on the right side before the boss Grisha, (before go down the stairs or go left along the corridor from the room where the fly agaric grows). You will be taken to a secret room with a sarcophagus. Glass is hidden in it.