Its in-game description reads "Ancient sword forged long ago." Unlike the Sand Breaker, it has a slow 5-hit combo, and its got good Flame, Lightning and Exorcism stats. You can find the Antique Sword in dungeon chests, or you can buy it at Jurak Arms in the Jurak Mall for 880 Gilda when you get to Chapter 5. It's a medium-tier weapon (Tier 5), meaning it will gain 4 Synthesis Points (SP) per level-up.