The Word Of God is an inscribed tablet that was excavated by Dick Roman in Iran. It was written by the angel Metatron, and includes instructions on how to kill the Leviathan. Castiel says that if the Word of God is revealed a prophet will be awakened and that angel protocol dictates that the prophet be taken to a desert to learn the Word away from men.
Charlie manages to get into Dick's office and download his emails. Back at her desk she sends these, along with the contents of Frank's hard drive to the boys. She discovers that a package for Dick is arriving from an archeological dig in Iran at a nearby airport and manages to send a fake email, so it appears to be arriving 30 minutes later. This allows Sam and Dean time to get to the airport and switch the package for a case with a borax bomb in it.
When they open the case they find it contains a slab of rock.
Dean cracks the stone containing the tablet. As he does so there are electrical storms, women go into labour, Kevin Tran becomes a Prophet, and Castiel wakes from his comatose state. What can be read from the translation is of the following:
"Cut off the head, and the body will flounder. Waste not thy time nor your breath upon the Leviathan herd. Point thy blade at the heart of their master, for from him springs all their messages. Leviathan cannot be slain but by a bone of a righteous mortal, as light and good as the Leviathan are hungry and dark, washed in the three blood of the fallen: a fallen angel, the ruler of fallen humanity, and the father of fallen beasts."
Sam and Dean decide to start gathering the needed parts of the weapon and start by getting the fallen angel blood from Castiel. When Kevin returns home and is captured by Edgar, he still has the tablet and it is captured with him.
After having captured Kevin Tran, Dick Roman forces him to translate the tablet for him. After the translation is done, Dick sends out Edgar to kill the Alpha Vampire and get his blood and summons Crowley into a Devil's Trap to make a deal with him.
At the same time, Bobby Singer figures out that the Ruler of Fallen Humanity is Crowley and that for a Father of Fallen Beasts they need an Alpha. Crowley is summoned and promises them his blood once they get the other needed pieces. Sam and Dean are able to get the Alpha Vampire to give them his blood after they save him from Edgar and prove that the Leviathans have betrayed him.
Sam and Dean steal the bone of a nun, Sister Mary Constant to get the "bone of a righteous mortal" while Dick makes a deal with Crowley to have him give them the wrong blood. However, Crowley informs them of the deal and gives them the right blood and they are able to make the weapon the tablet describes. Dean is later able to use it to kill Dick Roman with the help of Castiel.