This is story related and cannot be missed.
Near to the end of the game you will eventually reach the hourglass, after the cut-scenes that follow you will be in a mysterious room with about 6 doorways. The secret to this room is entering the doorways in the correct order.
TIP: To know the correct order, simply walk around the room slowly, as you get next to the doorway you need to enter you should here some water trickle, when you do enter the doorway & begin to find the next correct one.
Do this for the first and second level to get out of this room. After the next cut-scene you will be by yourself, follow the corridor passed the enemies to an outside section where a slab of stone is located in the middle. On this stone slab is your next sword but there is a barrier protecting it.
Located around this area is four mirrors on moveable stone, you need to move all four mirrors so they catch the light & line them up with more mirrors lined around the outside of the area. You want to line them up correctly so the last mirror reflects onto the strange symbol attached to the huge stone slab, that will bring down the barrier which will give you access to the sword.
TIP: Look out for the 4 darker areas on the floor in this section, it will give you an idea of where the 4 mirrors need to sit.