The Phurba is an ancient three-sided Tibetan ritual dagger used as a guide and key to locate and enter Shambhala. Marco Polo referred to it as the "golden passport" in one of his journal entries. Nate later found it in Borneo, after which the dagger, as everyone commonly calls it, frequently switches hands, until Nate finally uses it to open the final gate to Shambhala.
Prior to Among Thieves
Polo originally received the Phurba due to his close ties with Kublai Khan, who presumably acquired the item through his grandfather; Genghis Khan. Using the dagger he located the correct temple in a Nepalese valley, which ultimately led him to the discovery of Shambhala underneath a Tibetan monastery. The Phurba then has to be used as a key to unlock the first of two gates to Shambhala, which causes it to be gone as the gate will sink into the floor; the Phurba along with it.
Presumably Polo later reacquired the item when the Shambhala citizens closed the gate behind him again as he left, which lead to the dagger being ultimately lost for several hundred years when part of his fleet crashed in Borneo.
After finding the resting place of the sailors of the lost fleet of Marco Polo, Nate, Sully, and Chloe searched it during "Borneo". They found the Phurba, alongside a drawing depicting a valley filled with temples. They recognized the location, but also realized that the field of temples had long since turned into a city. Nate managed to hide the dagger's existence from Flynn when he captured them shortly after.
During "Urban Warfare", in the Nepalese city, Nate reunited with Chloe and realized that the symbol on the dagger would lead them to the right temple, which they spotted in "Desperate Times". In "The City's Secret" they entered it, and used it to solve a puzzle involving a large statue, itself also holding a large Phurba. Once inside the temple's largest hidden chamber, during "Path of Light", they spotted a gigantic Phurba hanging from the ceiling. Using the real Phurba as a key they had to lower the giant replica's blades, and ultimately plunge it into the chest of a statue on the floor. This revealed a ladder leading to a map with the entrance to Shambhala.
Later on in "Keep Moving", Lazarević took the artifact from Nate, handing it over to his lieutenant Draza. Nate took it back from Draza after his defeat in "Tunnel Vision", and subsequently temporarily lost in the resulting train crash. He reacquired it at the end of "Train-wrecked", as seen in the opening chapter "A Rock and a Hard Place". Schäfer took the dagger from him after Tenzin found him unconscious, and kept it safe while he urged Nate to discover the truth about the Cintamani Stone.
Lazarević's soldiers managed to steal the item again when they kidnapped Schäfer during their siege on the Tibetan village. Nate and Elena attempted to rescue Schäfer and retrieve the dagger, but learned that Lazarević had left the older man at near-death in the monastery. In "Reunion", Nate accidentally overheard a conversation between Lazarević and Flynn, with the former threatening the latter by stabbing the Phurba into a wooden post next to his head. He then left it to Chloe, who stayed behind alone. Nate then convinced Chloe to give it to him, and ultimately used it under duress to open the first gate to Shambhala in "The Road to Shambhala".
Considering the final events of the game the Phurba could be considered lost forever.
Behind the scenes
The Phurba, and a stand, are part of the limited Fortune Hunter Edition for Among Thieves.