Lunar Threads

I was looking through the TSS guidebook and I found an item that I had never heard of before. It was called the "Silver Nail". Jessica or Alex can equip it and gives +100 attack. Where can I find it?

It might have been named "Silver Claw" in the game. It's in a chest just outside the Marke entrance to the Black Dragon Cave. I'll check my notes when I get home to be sure.