The Rune Claws can be obtained in Memoria during Disc 4. You will climb some stairs and enter a room called the Time Interval room. There is a large staircase leading up towards a giant red eye. Move to the left edge of the screen just atop the stairs you came from and behind the first pillar from the left edge of the screen. You should see the ! field icon revealing the weapons location. The Rune Claws are Amarant's Ultimate weapon.