Serenade

 is a power skill/melee weapon which summons a friendly flying sword. It is exclusively obtained from the Fatal Falls DLC.

Location
In the Fractured Shrines there is an island that can be reached by platforming over invisible platforms; the rain splashes reveal their location. After that, there is a small gauntlet of traps that leads to a temple that can be entered either through the top 3 breakable floors or through the other side where a door is.

In the building, there is a Stone Warden guarding a door which leads to a unique treasure room with a vault inside. Opening the vault will free Serenade, forcing it onto a skill slot. The blueprint itself goes in your inventory and still needs to be delivered to the Collector.