U+A877 Phags-Pa Mark Double Shad

The Wikipedia has the following information about this codepoint:

The Phagspa script or ʼPhags-pa script is an alphabet designed by the Tibetan monk and State Preceptor (later Imperial Preceptor) Drogön Chögyal Phagpa (1235-1280) for Kublai Khan (r. 1264–1294), the founder of the Yuan dynasty (1271-1368) in China, as a unified script for the written languages within the Yuan. The actual use of this script was limited to about a hundred years during the Mongol-led Yuan dynasty, and it fell out of use with the advent of the Ming dynasty.

The script was used to write and transcribe varieties of Chinese, the Tibetic languages, Mongolian, the Uyghur language, Sanskrit, probably Persian, and other neighboring languages during the Yuan era. For historical linguists, its use provides clues about changes in these languages.

Its descendant systems include Horizontal square script, used to write Tibetan and Sanskrit. There is a theory that the Korean Hangul alphabet shows limited influence from ʼPhags-pa (see Origin of Hangul). During the Pax Mongolica the script even made numerous appearances in western medieval art.