Description
Inscribed into the blade of the sword are the words 'Swing madly'.
If broken it will not be repairable, repair with legendary weapon repair kit before it becomes fully broken.
Notes
This weapon only does full damage on light attacks, heavy attacks inflict only 1 damage.
This is the strongest 1-handed sword in the game.
Source
The The Predatory Blade can be found in 
the Unnamed City, by looting boss creatures. Dropped by the Brute boss.
Repair
This item can be repaired with a Legendary Weapon Repair Kit.