From Gamefaqs:
Name: Rune Blade
Info: "Weapon that has rune inscriptions"
Price: 8900 Gil
Sell: 4450 Gil
Attack: 57
Add ST: Darkness
Abilities: Iai Strike
Used by: Steiner
Location: Buy from Daguerreo (Disc 3)

Weird that it's not on our main site.