The item is used to upgrade the Blade of the Great Martyr and unlock the first skill. We'll tell you where to find it.
In the abandoned halls where you unlock the Blade of the Great Martyr, go deep into the location. Before the Enslaved Grisha boss, on the right side, look for a hole in the wall (before going down the stairs or going left down the corridor from the room where the fly agaric grows). You will enter a secret room with a sarcophagus. It has glass in it.