Stake

A Cypress stake is a stake made from the branch of a Cypress tree found in Babylon. It's the only thing that can kill the Whore of Babylon. However, it must be wielded by a true servant of Heaven in order for it to work.

In the Season 5 episode "99 Problems", Castiel acquires a Cypress stake from Babylon to use against the Whore, who is trying to condemn the inhabitants of a town to Hell. Neither Castiel nor the Winchester brothers consider themselves to be true servants of Heaven (Castiel and Dean have both openly defied Heaven, and Sam is an "abomination"), so they give it to the town's priest Father Gideon instead. However when the group confronts the Whore of Babylon, Gideon drops the stake in the struggle that ensues. Dean picks it up, and although both he and the Whore are convinced it won't work, he stabs her though the chest with it. Surprisingly, it works and the Whore is killed.

The fact that Dean was able to kill the Whore - thus qualifying as a "true servant of Heaven" - makes Sam worry that he's going to give in to the Archangel Michael.