Serenade

 is a power skill 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 blocked by three layers of breakable floor thus requiring the to get inside. 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 and they'll force theirself into a skillslot. The blueprint itself goes in your inventory and still needs to be delivered to the The Collector.