In Chapter 1, leave the room with the wizard and ascend the ladder. Move all the way to the right to the altar, then step on the symbols in the correct sequence. To learn what that is, step back slightly and look up at the stars. Successfully entering the sequence will unlock a grate. Go to the tower and enter the room where that grate is found. Once there, you will find a wall with a large hand on it. The chainmail is stuck between the stones in that wall. Keep pulling the chainmail until it comes out. Note: Before unlocking the grate, the chainmail will be visible but cannot be taken out from the wall.
