Metatron

Metatron is the angel whom, according to Castiel, wrote down the Word of God when creation was being formed. The Word of God was inscribed on multiple stone tablets in a language that most beings - including angels could not read. One of these tablets containing the Word had instructions on how to kill the Leviathans should they escape from Purgatory.

Trivia
Since he wrote the Word of God at the beginning of creation he may be older than the Leviathans, although its never stated which creation he was at the beginning of.

The Word of God which gives the methods of killing Leviathans was found in Iran.

It appears the Word of God can be restore or fixed to it's original state by the Prophet linked to it. As when Kevin Tran fixed the broken Word of God, when he brought the pieces together.

The Word of God also has numerous affects. As when Dean removed the tablet from the clay, every pregent woman in 100 miles went into labor, caused storms, alerted the remaining Angels that it was found, and awaken Castiel from his coma. (However, these affects seemed to only have happened after, the talbet was freed from what Castiel called "The Vault of the Earth.''