In secondary mode, consecutive weak attacks with this weapon continuously increase in speed. These rapid attacks deal a lot of damage quickly but will quickly drain stamina at the same time. In the transformed state, L2 slices while simultaneously jumping back.
Gems that add arcane damage will boost the elemental effectiveness of this weapon.
Gems that add attack damage, not physical attack up, will also increase the Arcane damage this weapon does.
Each blade of this weapon has a completely smooth face so it can stick to the other much better.
The Blade of Mercy is forged from siderite (Iron Carbonate, FeCO3), which is magnetic when heated. Coincidentally, the Burial Blade is also made of siderite. This makes it possible that Gehrman was the one to design it.