The Great Treasure Endowment of Fire Lord Mokutan (by the Fire Nation contemporaneously and until the last century), also the Great Treasure (大寶) Donation (more neutrally), was a massive donation of land to the Western Air Temple by the Fire Nation and Earth Kingdom. It was part of a peace settlement between the Fire Nation and Earth Kingdom, enforced by the particularly powerful Jiongyu Avatar (1181-1122 BG, Fire.) It was finalized in 1140 BG, in treaty agreed to by the governments of the 4th Earth King (Ting Dynasty), the Fire Lord Mokutan, the Western Lama, Yeshe XII, and the Chiefs of the Northern Water Tribe (who, besides agreeing to help protect the terms should a party break the treaty, also had an interest in protecting the rights of the Water Tribe minority who had been the first to arrive on the Northern coast of the affected lands.)
See Avatar/Western Air Temple for further details.