"Increases your movement speed for 5 seconds after killing an enemy."
Notes
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.
The curse effect can be temporally disabled by using the Serenade in as a weapon.
Dying while using the Serenade as a weapon will still counts as death by curse.