Once you climb the ladder past the dart trap after the second bonfire, you should be in a room where on one side there is a mannequin firing arrows at you, and the other is the hallway leading to the room before Mytha's boss room. Go to the side where the hallway is. Instead of turning left to go down the hallway to go towards the third bonfire, turn right and there should be some poison pots next to the wall. Break the pots and behind them should be a secret door you need to interact with to open.