When you have all the Elemental Reavers, go to the castle where you began. Use the Fire Reaver on the floor. It has the sign of the Fire Reaver and is orange. Plunge the sword into it, then get the light. Use it on the door, then proceed until you get to a room where the Soul Reaver is located. Mobius will appear and insult Raziel. He will then take the Soul Reaver and be invincible. Note: You will suck blood instead of souls and also fight all Raziel's brothers and Raziel when he was human.