Death's Scythe

Death's Scythe is the personal accessory of Death, the eldest of the Four Horsemen of the Apocalypse. The scythe is a nod to popular belief that, as the Grim Reaper, Death carries a scythe that he uses to reap the souls of living beings. Being of Death's own design, the scythe naturally has the power to reap/kill any living being it is used on, even rumored to be capable of killing even Death himself (though this remains unconfirmed). In the context of the show, the scythe is not initially in Death's possession, though it is eventually made clear he was the creator of the weapon and that it is his property by the demon Alastair. The scythe is eventually returned to the hand of its master unwittingly by Dean Winchester when the two meet face-to-face for the first time.

Powers
Death's Scythe is arguably one of the most powerful and potent weapons mentioned on the show, as the demon Crowley notes that it is capable of killing Angels, Demons, and possibly even Death himself. This would make the scythe even more powerful than the blades of the Archangels or the Colt, as the two latter are suited to only limited usage, meaning that the scythe may be the only weapon in the entire Universe that truly does have the power to "kill anything". The scythe will also burn red-hot in the presence of its master, Death, if it is not in his possession.

Death Takes a Holiday
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 captivity by the Angels, it is unclear what became of the scythe (though it would later resurface).

Two Minutes to Midnight
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 restored 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.