"Increases your movement speed for 5 seconds after killing an enemy."
There is an achievement associated with this weapon called "I like to live dangerously...", which is achieved by completing a run with the sword in possession.
The weapon's curse mechanic remains active even if it is merely held in the backpack (and therefore technically not "equipped" in the usual sense). The curse effect can only be escaped by dropping the weapon entirely.
Unlike a curse from a cursed chest, equipping it does not prevent the player from using Homunculus Rune and does not trigger the crit damage condition of Spite Sword.
Ygdar Orus Li Ox will not be triggered when killed with this weapon in your inventory.