Dark Matter. You can buy it in the Auction House in Treno or steal it from Ozma. I think you can steal it from Ozma. I believe there's an ability that raises his strength (as there's only two dark matters in the game). Odin's Sword or something. I think Ancient Aroma teaches it. I'm pretty sure that's found somewhere in Ipsen's Castle.

Edit: Beat me by seconds. I'm pretty sure you can still buy the Dark Matter on disc four, by the way.

Edit 2: I checked up on Ozma and it seems you can get two dark matters off of him. You can steal one, and one is a guaranteed drop after winning.