Once you defeat Tiamat in the floating fortress go to Wisperwind Cove. You have to go through three bosses (Typhon, Orthros, then the Phantom Train each after 10 floors). Keep going till you to the 40th floor and beat Death Gaze. Step on the warp panel then when you get to the next look for a door to your right near the second warp and go through the door. Open the treasure chest to get the Ultima Weapon. Any higher class character can use it.