The Sundom is the domain of the Carja tribe in Horizon Zero Dawn. The Sundom is lead by the tribe's ruling monarch known as the Sun-King, a title currently held by the Radiant Avad, 14th of the Line of Luminance of the Carja royal family.
The Sundom is currently embroiled in a civil war, with supporters of Avad defending Meridian, the capital city, against those who supported his late father, the Mad Sun-King Jiran, and have since been exiled to Sunfall.
The first Sun-King Araman led a pilgrimage east from the Sacred Lands and established the city of Meridian. The surrounding area was developed into farmlands and, eventually, housing for villagers and nobles. However, it was not until the third Sun-King Sadahin came to power that the land was established as a territory and designated "the Sundom." Sadahin expanded the border of the Carja's land by establishing a port on the southern shore of the Daybrink and similarly erecting gates to south and east of Meridian.
The kingdom's boundaries were again expanded during the reign of the fifth Sun-King Zavarad, who crossed the Great Lake of the Daybrink and established the port known as Blazon Arch on the northern shore, thus claiming the vast body of water for the Carja.
Basadid, the seventh Sun-King, cemented the western border of the Sundom by ordering the construction of a large military base called Sunfall and declaring the land to its west forbidden to members of the tribe. Khuvadin, his successor, established the eastern border after failed attempts to colonize the tribesfolk that remained in the Sacred Lands convinced him that fortification against their territory was necessary to protect the Carja.