Lefty's Brass Knuckle

Lefty's Brass Knuckle is a fist weapon quite nice for Rogues as it is a One-Hand weapon and not Unique. This allows them to equip two at once for a fast attack speed and a healthy Agility bonus.

Source
Upon killing Theldren in BRD, the Arena Spoils chest will spawn. This weapon is one of the possible rewards from that chest. Theldren can be summoned in the Ring of Law using the Banner of Provocation and is part of the Dungeon Set 2 quest chain.

This item is the weapon of Lefty, one of Theldren's gladiators and a possible opponent in the fight against Theldren.