The Upali Thein (Pali: Sima) is a small building lying along the main Nyaug Oo - Old Bagan road, which could easily be overlooked. Even one of the bigger books about Bagan art and architecture (Pagan by Paul Strachen) doesn't list it amongst the more than 70 monuments it covers.

The Sima was erected during the reign of King Kyazwa, who reigned between 1234-1250, and was known as a philosopher King. The Glass Palace Chronicle has this to say of the good King (translation adapted from that by Pe Maung Tin):

He had compassion on all the people, both laymen and monks, as though they were children of his bosom. He read the Three Pitakas nine times over. He pondered often on divers interpretations of the Pali, commentaries and sub-commentaries; in the discussion of questions there was none to equal him. Seven times a day he studied with the noble Order.

For the sake of his concubines he composed [an Abhidhamma work] the Paramatthabindu, that they might know of mind and the qualities of mind, matter, nirvana, forms of being, and personality. He would not even lend an ear to affairs of the villages or kingdom. Whenever there was any inquiry to be made, power exercised, or point of law determined, he caused his son Uzana, the heir-apparent, to dispose thereof.

Ven Upali is named in the Glass Palace Chronicle as Mahasiha-Upali, and was initially invited to Bagan by King Nandaungmya, Kyawza's Father, who built a monastery for him and some other famous monks of the time. Presumably the rest of the monastery was wooden and has now perished, but the Sima has survived.

The Sima was renovated during the reign of the Konbaung Dynasty in the late 18th century, which is when the mural paintings date from. They contain scenes of the Going-Forth of many of the previous Buddhas; other famous scenes from the Life of the Buddha, like his son's Going-Forth and Ajita's confirmation as the coming Metteyya Buddha; the Rains' retreats; and the consecration of the Sima.

The building was badly damaged in the 1975 earthquake and has been roughly shored up to prevent collapse, though some of the paintings have been badly affected by the quake. Flash photography is not allowed to prevent further damage, but there was just enough light to get photographs of the murals, which I have had to adjust to make clear.