Rune Parchmentルーンの羊皮紙

An unusually well preserved piece of ancient paper. The runes are impossible to decipher but small symbols hint at it being the recipe for a strange weapon.
In-game description

How to use Rune Parchment

Rune Parchment is used to craft Rune Song. You do not need a crafting station: you use Rune Parchment itself.

Have the materials in your inventory (Broken Handle ×1, Chipped Blade ×1, Clear Gemstone ×1, Iron Bar ×50, Ancient Gemstone ×10).
Put Rune Parchment in your hotbar, select it and use it (right-click). Using it takes 10 seconds.
Rune Song is crafted.

How to collect the parts for Rune Song is on the Rune Song guide.

Rune Parchment key points (from the game data)

Sells to merchants for 400 Ancient Coins.

Calculated and summarized by this site from the game data (1.3.0.6).

What is Rune Parchment used to make? Craftable items and their uses

Items you can make with Rune Parchment, with how to craft them, the other materials needed and what the result is for.

Rune Song

How to craft: Hold Rune Parchment and use it (no crafting station needed)
Materials: Broken Handle ×1, Chipped Blade ×1, Clear Gemstone ×1, Iron Bar ×50, Ancient Gemstone ×10
What it is: A legendary sword (melee weapon). Rune Song: stats and special attack

The game data has no recipe or source for this item.