Death's Scythe

Death's Scythe is a personal belonging of Death's, which he himself created, to kill any being in the whole universe.

Powers
This weapon is the for sure the most powerful and potent weapon to appear on the show, exceeding an Archangel Blade, The Colt, and Ruby's Knife.

It has been described to be able to kill Angels, demons, and Reapers, however as it is of Death's own making, it's inferred that it can kill anything.

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 custody 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.