Black Devil's Scimitar

Black Devil's Scimitar (黒悪魔罰当り偃月刀, Kuro Akuma Engetsutou) is a Black Devil Slayer Magic spell that allows the user to incorporate darkness into their sword.

Overview
This spell is a relatively simple one in the plethora of Black Devil Slayer spells, though it is still very effective. To use the spell the user must first have a knife or sword in their possession. To initiate the spell they must focus their shadowy energy into their hands as they grip their sword. Their hands will slowly turn black from the shadows gathering within them. Then, darkness will steadily flow from the user's hands into the blade, causing the weapon to slowly morph into a large broadsword. The sword itself however does not gain weight as it gets bigger, as the mass it gains is pure shadow and thus, does not add weight to it.

Additionally, the user must constantly hold their grip on their sword and feed it their dark energy in order to maintain the spell. This causes the user's hands to stay black through the entirety of the spell's casting. If the blade is somehow thrown from the user's grip or they drop it, it will immediately revert back to it's original state. This spell is especially useful if the user's original sword were to break in battle, as they only need the hilt of the sword as a base to fully activate the spell.

Trivia

 * The sword that the user creates with this spell is actually nothing like a real scimitar, but scimitar sounds so much more kickass than just plain old sword.