Rustbitten Dirk is a Unique Main Hand in Diablo IV Season 15 for All classes. It comes from General Unique pool from any Lair Boss; Random drops / Horadric Cube Unique upgrade of a common dagger; General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table (one guide's Boss Loot Table Cheat Sheet).
Unique power
Deal [90 - 120]%[x] increased damage to isolated enemies.
Affixes
Weapon Damage
All Damage Multiplier
Where it drops
General Unique pool from any Lair Boss
Random drops / Horadric Cube Unique upgrade of a common dagger
General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table (one guide's Boss Loot Table Cheat Sheet)