First seen in Day 52.
A rare, valuable magic metal. Normally too precious for a goblin to have.
A Mithril knife does not contain any special abilities, but its sharpness is beyond compare to the standard steel short sword that they are currently equipped with. Mithril can easily cut the blade of a steel sword without getting a scratch on it.
Only the elves have the ability to manufacture the Mithril Knife. It's so (Rare) that a low level adventurer could not obtain one.
Rou gave mithril knives to several departing older generation goblins as a farewell gift when they decided to leave Parabellum. Even with the Mithril knife they still have a high chance of being killed by a group of adventurers or mercenaries. With the market value being so high, a treasure like that usually invites trouble.