User blog comment:Fusedtwilight/Possible Season 11 Plots/@comment-92681-20150523064955

They said the same thing about the Leviathans not being "killable" but Dean, Sam and Cas and Bobby found the weakness.

I think The Darkness should have some obscure weakness or something, maybe that Spear of Destiny that Dean found in the Bunker a few seasons back, that has to be useful for something.