The First Prophecy

It was a happy day when the construction of the Great Oracle was finally finished after 300 years. As part of a grand celebration, the first Heralds of the Thirteen would—for the first time ever—be seated upon their thrones to finally be reunited with the gods and receive their gratitude for the construction of the Oracle. Instead, the lights flickered out and thirteen voices echoed as one from all sides of the main hall in a deep and awe-inducing tone:

With the city's return when all splinters are taken,
once the oracle speaks the seven will waken.
Then, the storms come in crashing and the earth starts to shake.
Once our dragons all sleep the barrier will break!
Go gather your forces, you'll have to defend!
If all of this happens this world soon will end!

Only seconds later, all candles lit themselves again and shed light on the faces of the panicked audience. Even though only the Heralds speak the god's language, everyone could grasp that this was not a cheerful greeting. The celebration was ended before it started properly while the Heralds retreated to discuss the meaning of this prophecy.

Consequences

Only days later, the Oracle and the area around it were sealed for the general public as the realisation struck that the eternal war of the gods was not over but had indeed only just begun. And after all, archived prophecies could just as well be used as an instruction manual by the followers of the Seven on how to thwart any righteous plans communicated by the Thirteen.
Walls were constructed around the oracle and the Silver Guard was founded to protect them and everyone within. Over the following years, able men and women from all over the world were recruited for this task. The most able and trustworthy among them were tasked with finding followers of the Seven and bringing them to justice. This was the beginning of what we today know as the Inquisition.

Spread

The prophecy itself was immediately sealed away after it was written down, for the safety of everyone. Only the most important individuals in the churches are allowed to see it. This includes all Inquisitors as it is their task to spot any signs of the prophecy becoming true. The poem has become their unofficial motto and is engraved into marble above the entrance to the Inquisition's headquarters on the Island of Gods.
Outside this select group of people, the knowledge of the prophecy is indeed obscure. A few more knowledgeable historians might know of its existence and maybe even its rough context, but not the elements mentioned in it or even the exact wording. It is one of the best-kept secrets of the world.

Interpretation

The exact meaning of the prophecy is still subject to discussions and interpretation. Especially the first verse has been a mystery. There are as many theories about to which city is referred to and what the meaning of "returning" is as there are about the nature of the mentioned splinters. Some say the second line refers to a separate Oracle to be built by the followers of the Seven, others believe it refers to an upcoming prophecy by the Seven. Many fear, the true answers will only be found once it is too late.