The Yuan Dynasty celadon oil lamp was a gift from Kublai Khan to Marco Polo following his travels through the Mongolian Empire. The simple piece of pottery was covered in Mongolian script and was on display at the Istanbul Palace Museum as part of their Marco Polo exhibition.
Whilst being stored in the Istanbul Palace Museum, it was stolen by Nathan Drake and Harry Flynn. Although the lamp never left the exhibit, it was broken by the two thieves to reveal its secret. Hidden inside the lamp was a simple, and apparently blank, piece of parchment, browned by the age it had spent in the lamp, and several chunks of resin.
Drake, realizing that the resin would burn, lit the small pile on the floor and revealed the secret of the parchment. The "light of the Great Khan" revealed the otherwise invisible marks left on it by Marco Polo.
Concealed on the parchment was a map of Indonesia and Borneo, showing the location of the "Lost Fleet" on the west coast of Borneo and revealing, for Drake, the true value of Marco Polo's information. Unfortunately for Nathan Drake, it also sealed his fate for the next three months. Betrayed by Harry Flynn, he spent the time in a prison in Turkey until he was bailed out by Victor Sullivan and Chloe Frazer.
Marco Polo's information was indeed accurate and the Lost Fleet was found on the west coast of Borneo, but, although many artifacts were recovered, neither the Cintamani Stone (as Nathan Drake believed) nor the next clue to lead to Shambhala (as Zoran Lazarević believed) were found with the ships.
After 3 months in prison, Nathan arrived in Borneo with determination to succeed where Flynn had already failed. By traveling through the jungle over rough terrain, and only a few hundred meters from the main camp, Drake and Sully came across the remains of the 600 or so passengers who traveled with Marco Polo on his voyage, and with help from Skelzor, found the next clue, which led to the Valley of Temples in Nepal.