The Mongols left but the Shan people, who had come down with the Mongols did not—just as the Burmans who came down with the Nanzhao invasions stayed behind four centuries earlier. The Shans built an array of small states in the entire northwestern to eastern arc of central Burma, thoroughly surrounding the valley. They continued to raid the Irrawaddy valley throughout the 14th century, taking advantage of the split of Myinsaing into Pinya and Sagaing kingdoms in 1315. Starting in 1359, then the most powerful Shan state of Mogaung  began a series of sustained assaults on central Burma. In 1364, its forces sacked both Sagaing and Pinya in succession, and left off with the loot. The power vacuum did not last long. In the same year, Thado Minbya, a Sagaing prince, emerged to reunify Upper Burma and founded the Ava Kingdom. Nonetheless, Shan raids into Upper Burma continued off-and-on in spurts. The raids were led by different Shan states at different times. Mogaung's devastating raids  were replaced in the 1370s and 1390s by Mohnyin's raids that reached as far south as Sagaing. After Ava conquered Mohnyin in 1406, the mantle was picked up by Theinni  which raided Avan territory from 1408 to 1413. The intensity and frequency of the raids lessened in the 15th century due to both the rise of Ava and indeed the arrival of the Ming Chinese in Yunnan in the 1380-1388.
Question: Which group did not leave, Mongols or the Shan?
Answer: Shan