The Golden Bull of Rimini was a document issued by Emperor Frederick II of the Holy Roman Empire on behalf of the Teutonic Knights. It ceded land to them which didn't belong to Frederick's Empire in the first place in return for the conquest of the same land and the conversion of its pagan population to Christianity.
A bulla aurea (Latin for gold seal) was a gold seal attached to a document. Over time, the description of the seal came to mean the documents sealed in that way. Golden bulls or chrysobulls were customarily issued by the Byzantine Emperors for land deeds and the conveyance of privileges. Medieval European rulers adopted the custom for especially solemn occasions.
Golden bulls were issued rarely and they stand out like beacons in between all other decrees. Frederick II issued one in 1235 which became known as the Golden Bull of Rimini. The bull in itself was straightforward enough: It ceded the lands of Chelmno (Kulmerland) and Prussia to the Teutonic Knights in return for their efforts to conquer these lands and convert the pagan inhabitants to Christianity. There are, though, several intriguing twists to this document and its content.
The bull was issued in 1235, but dated back to 1226. This doesn't mean that the content of the bull was laid down in 1226 and then written down later, the dating is due to political aspirations; ‘aspirations’ is used in plural because there were aspirations of three European rulers: Duke Conrad I of Masovia (and from time to time High Duke of Poland), Pope Gregory IX, and Emperor Frederick II.
The muddle was started by Conrad when trying to conquer the Kulmerland from the pagan Old Prussians to add it to Masovia. He failed abysmally, and in return the Old Prussians went into the offensive and effectively threatened the integrity of Masovia. At that point in 1226, Conrad called on Grand-master Hermann von Salza and his order of Teutonic Knights to come and help him.
The Grand-master Hermann von Salza's backside was still smarting from the beating he had received in Transylvania. His order of battling knights had been ousted from Transylvania the previous year by King Andrew II of Hungary. The latter had reneged on his promises to the order for independence from the Hungarian crown and annexed Transylvania. When the call from Masovia came, the Teutonic Knights didn't move a finger for Conrad but stated their demands for independence and asked for a promise in writing. Conrad gagged on that and founded his own order of knights instead: The order of Dobrin. With them as part of his army, he was handed a further crushing defeat by the Old Prussians.
In 1230, Duke Conrad caved in and signed the Treaty of Kruszwica ceding the Kulmerland to the Teutonic Knights. The original document of the treaty is not in existence anymore (or has not been found to date) which makes Polish historians claim that it never existed or was a fake made by the Teutonic Knights to annex Polish lands. They conveniently overlook two historical facts in their claim (and are faithfully cited by historians all over the world for their pains).
The most glaring fault in their reasoning is the fact the Kulmerland was not Polish; it was land held by the Old Prussians and Conrad had failed to make it Polish. It was and remained independently held by the pagan Old Prussians; no Christian rewriting of history can prove otherwise. And the Teutonic Knights moved into the Kulmerland in 1231; they wouldn't have done that without the necessary guarantees as to hold what they conquer. Hermann von Salza had learned his lesson in Hungary and he was anything but stupid. But it makes one of those beautiful Polish legends that go for history there. Presenting a fraudulent document signed by a person still alive would have been rather brazen, too; just because they didn't have mobile phones doesn't mean rulers didn't know what was going on at their respective courts.
In fact, Conrad was a fraud; he ceded lands that didn't belong to him to the Teutonic Knights. He was soon joined in deceit by more illustrious fraudsters, though. In 1234, after the Teutonic Knights had already started building the Castle of Thorn, Pope Gregory IX issued his Bull of Rieti ceding the Kulmerland and all of Prussia to the order of the Teutonic Knights as lands belonging to the church and not being in fiefdom to any worldly ruler. His hope was to keep the newly Christianized land under the direct influence of the Pope without interference from any of its neighbors.
Issuing his Golden Bull of Rimini in 1235, Frederick II had little compunction of ceding the same lands again. The Duke and the Pope both had already ceded lands not belonging to them; he therefore did the same again, and added the little fraud of the date on top to make it clear that his claim came first. God wouldn't mind, it was all for the best of the pagan Old Prussians who virtually cried out to be converted to Christianity.
The result of all this was a bloodbath. The Old Prussians were so desperate to be converted to Christianity that they preferred to be put to the sword rather than give up their pagan religion. The pitiful rest of the population that had survived conversion by the sword was soon swamped by German colonists swarming over the lands like ants. If ever there were true martyrs to their Faith, it was the Old Prussians. And I am sure God knows that.