Death's Scythe

Death's Scythe is a personal belonging of the horseman Death. It is arguably the most powerful weapon in Supernatural. After being used to kill Death himself, it crumbled to dust.

Physical Appearance
It appears that the scythes appearance can be altered at the will of Death. The first time the scythe appeared, the instrument had a short, wooden handle and a black, possibly iron blade. Technically, this version is a sickle; a scythe has a longer handle and blade, as opposed to the fist-sized handle and the half-circle blade. When Death gives Dean his scythe in Brother's Keeper it appears more like the mythical version rather than the previously seen one.

Powers

 * Killing - Death's scythe can kill any living being. It can even kill beings that have been said can't die, such as horsemen. While it can kill the horseman himself, it won't destroy its attribute.

Season 4
The scythe is first seen in the possession of the demon Alastair when he attempts to use it to kill two reapers in order to break one of the 66 Seals to release Lucifer from his cage in Hell. He notes that he "borrowed it from an old friend" who "doesn't really ride a pale horse, but he (Death) has three amigos, and they're just jonesin' for the Apocalypse!" After Alastair is taken into custody by the Angels, it is unclear what became of the scythe (though it would later resurface).

Season 5
Having discovered Death's location in Chicago, Illinois while on a quest to retrieve the remaining rings of the Four Horsemen of the Apocalypse and return Lucifer to his cage in Hell, Dean Winchester is faced with getting Death's ring away from him against seemingly impossible odds (given Death's power). To help in this matter, Crowley reveals he is in possession of the scythe (having gotten hold of it through a crossroads deal) and gives it to Dean, telling him of its awesome power in the hope that it would give him a fighting chance against Death's power. However, when Dean tries to sneak up on Death in a pizzeria in Chicago, the scythe burns red-hot in his presence, forcing Dean to drop it. It vanishes and reappears on the table where Death is sitting, now returned to its master. Death thanks Dean for returning his scythe and then invites him to join him at the table to discuss the current situation of the Apocalypse being orchestrated by Lucifer.

Season 10
When he insists that Dean kill Sam in order to stop him from getting Dean back from where Death sends him, Death gives him his scythe to do it. Dean nearly kills Sam, but instead turns the scythe on Death himself, impaling him through the side with it, killing him. Death crumbles to dust with his death and the scythe with him.

Trivia

 * In Reading Is Fundamental, there was a scythe at Rufus's cabin that was identical to Death's Scythe.
 * Despite being in the possession of Hell during the ushering of the Apocalypse, the Scythe (along with the Colt) was only used offensively against angels, when Alastair killed a reaper with it.
 * The scythe that appeared in season 10 is clearly different than the one that appeared in season four and five, which is probably just because the writers wanted to improve how it looked.
 * Additionally, that scythe in the season 10 finale was also the same scythe that appeared in the video "Dave vs. Death".
 * As Death's Scythe is an extension of Death and his own powers, it would make sense that it could be used against Death as the only thing to kill him as it would be his own powers used against him. A similar concept would be if Death wanted to kill himself, then only his own powers could do so, making his Scythe a tool he could use to end his own existence.