Mischief is a dane-axe which a tier 3 weapon in terms of statistics. It can be acquired after the quest Lord of Seals. You can either kill Steinn the Clever during the fight for Orkneyjar to obtain it or you can buy it from him after the fight. He is selling it for 1,000 valuables, but with 2 diplomacy he can be convinced to sell it for 500. Even if you don't need the weapon, it can be recycled for salvage worth more than the buying price, so it's considered a good bargain.
The special property gives the wielder a 5% chance to deal an extra attack, but this is applied only during normal attacks.
This weapon can't be given back to Grimulfr later for any reward when you encounter him in Skjern.