Demise's Sword

"I've let a mere boy make a fool out of me for the last time. You stand before a demon... or should I say, a weapon without mercy!"

- Demon Lord Ghirahim

Demise's sword is the blade wielded by Demise in the finale of Skyward Sword. It is Ghirahim's true form; as the servant of the Demon King, and he has the capability of returning to the blade, like Fi and the Master Sword. It bears an upside-down Triforce on the blade as well.

Story
Throughout Skyward Sword Ghirahim pursues Zelda in attempt to revive Demise from his imprisonment. As Link progresses farther in the game, Ghirahim's appearance begins to gain dark marks in several places. In the final stretch, in order to fully revive Demise, Ghirahim takes on his true form which bears great resemblance to Fi, strongly emphasizing that he is in fact the spirit of a sword. When Demise is fully revived, he summons the sword from Ghirahim, his true form, and Ghirahim is sent into the sword. In the end, when Demise is defeated, the sword dissolves, likely taking Ghirahim with it.

Size, Abilities, and Characteristics
The blade is significantly larger in comparison to the Master Sword. It is capable of sending Link flying backwards in one strike and when lifted in the air during the presence of a severe thunderstorm, it can absorb lightning bolts to charge up and send electric shocks when stricken with a metal object, as well as beams.

Design
Demise's sword is almost completely black with a red aura surrounding it. The hilt bears resemblance to the Master Sword, with a red jewel as opposed to the Master Sword's golden one. The blade differs from the Master Sword in terms of shape and size, but it bears an upside-down Triforce emblem.

Theory

 * The design and shape of the sword bears a resemblance to Ganon's dual swords in Ocarina of Time. It can be inferred that later on in the timeline, the sword is powered up akin to the True Master Sword, thus the golden aura/color seen in Ocarina of Time, and another one exists through unexplained ways. Ganondorf/Ganon having hold of the Triforce (or part of it) at this time may be responsible for the golden color of the swords. Also, as the handle length increases in time, it is plausible that Demise's sword eventually becomes the Trident from Four Swords Adventures, as the middle prong is oddly swordlike and they are both weapons of great evil.

It can be inferred that this "Evil Master Sword" is inherited by eras of Demise's descendants and forms, just as the Master Sword is inherited by descendants of Link.

Trivia

 * Demise's sword is sometimes referred to as the Anti-Master Sword as well the Dark Master Sword by fans.