The monastery is a ancient Tibetan complex located high up in the Himalayans, featured in Among Thieves. It contains the secret entrance to Shambhala, which was what Zoran Lazarević and his soldiers were searching for. Nate and Elena were looking for the same thing, while simultaneously trying to rescue the kidnapped Karl Schäfer.
It is unclear when and by whom the monastery was originally built, although its purpose as a cover-up seems to be the main reason for its existence due to the inhospitable terrain. It is also unknown what happened to the original inhabitants, although the remains of a few corpses can be found, wearing similar garbs and with the same crossbows as to the villagers from the nearby Tibetan village.
Lazarević used Nate's marked map to pinpoint the secret entrance, taken from him in "Keep Moving". His private army raided the Tibetan village on the way there, during which they kidnapped Schäfer, also taking the Phurba. After the events in "Convoy", Nate and Elena witnessed Lazarević taking Schäfer deeper into the monastery, and slowly carved a path through the soldiers, inadvertently destroying a tower and a massive rope bridge into the main complex during "The Monastery".
During "Reunion", they eventually found Schäfer close to death, after Lazarević's torture. He confided in Nate the need to stop Lazarević from acquiring the Cintamani Stone. Nate and Elena then split up, with the former heading for the tower to attempt to reacquire the Phurba, and the latter looking for the secret entrance. On the way to the tower Nate witnessed a group of soldiers barely managing to force a pair of Shambhala guardians to retreat, and also saw several glimpses of the guardians darting across rooftops. He also overheard a large group of soldiers talking fearfully of being torn apart by the guardians.
Arriving at the tower, Nate spied on an angry Lazarević berating Flynn for being unable to solve the tower's puzzle, while Chloe stood by passively. After the two men left he dropped in on Chloe, and took the Phurba back, after which she left as well. Alone, he solved the puzzle with the help of the Ahnenerbe booklet left behind from Schäfer's expedition, which revealed the next clue to be in a nearby statue-filled area. He called Elena and they reunited again, and subsequently revealed the true secret entrance. They made their way back through the monastery and descended down the steps.
In "The Road to Shambhala" Nate quickly managed to solve the first puzzle, but the two were quickly caught by Lazarević himself. Threatening to kill either Elena or Chloe Nate caved in under the pressure and opened the first door, then cleared the way along with Flynn for Lazarević and his men. Before they could meet up they were attacked by two guardians, which resulted in Lazarević killing one of them himself, and revealing that it was actually a humanoid in disguise. Nate then used the Phurba to open the final door, granting them all access to Shambhala.
The location appears in different forms in multiplayer. It features as the setting of a Co-op Objective mission, entitled "The Sanctuary". It is also the basis for the The Sanctuary map, and the revised Sanctuary map in Drake's Deception.