Effective Against
LeviathanThe "bone of a righteous mortal washed in the three bloods of the fallen" is the only known weapon capable of permanently killing a Leviathan. This complex weapon was discovered through research into the Word of God tablets — the divine instructions left by God for dealing with various supernatural threats.
Components
The weapon requires: the bone of a righteous mortal (a nun or other person of genuine virtue), washed in three specific bloods representing different types of "fallen" beings. In the show, the three bloods were: the blood of a fallen angel (Castiel), the blood of the ruler of fallen humanity (Crowley, King of Hell), and the blood of the Alpha Vampire (the first fallen monster, representing the corruption of God's creation).
In Supernatural
This weapon was assembled and used in the Season 7 finale to kill Dick Roman, the Leviathan leader. The process of gathering the components took several episodes and involved considerable sacrifice. When Dean stabbed Dick Roman with the bone weapon, it destroyed the Leviathan leader but also created a shockwave that pulled Dean and Castiel into Purgatory as a side effect.