Cypress Stake
From Super-wiki
Characteristics
A stake of a cypress tree from Babylon is said to be the only weapon capable of killing the Whore of Babylon. However, it is only effective if wielded by a "true servant of Heaven" against the Whore.
Episodes
5.17 99 Problems
Castiel gives Sam and Dean a stake made from a Cypress tree from Babylon in order to stop The Whore of Babylon. The only catch was that it had to be used by a "true Servant of Heaven" to be effective. At first, they believe this to mean Pastor Gideon. However, it ends up being Dean who drives the stake through her heart and successfully kills her.