In the latest episode of Supernatural Dean
Stabs the darkness with a silver knife, which has no effect.
Previously Sam and Dean have spent time trying to find a way to kill the darkness but could not come up with any ideas. Is there a reason they have not thought to try using
Deaths scythe, which should be in Deans possession because he used it to kill Death. I have considered that it got left behind as the darkness was released straight after, but even so he should still know where to find it. Death once claimed that he would one day reap god and if the scythe can kill god then it stands to reason it could kill the darkness. Sure Death could have been lying but it's got to be worth a shot?