Assassin Oney is Named Enemy in Borderlands 2. They are located in the Southpaw Steam & Power region on the Planet Pandora. You encounter them during and after the side quest “Assassinate the Assassins” and in Raid on Digistruct Peak.
You can farm Assassin Oney for the Emperor as he is the dedicated loot source for this item. Assassin Oney’s legendary item has a 20% drop chance but only 5% on the Digistruct Peak.