The exact origins of Karseius I. are unkown. He most likely decended from a minor warlord in western Near Harad whose name is also unknown.
Nevertheless he founded the so called Karsassid Dynasty which would proove to rule Near Harad over several periods.
Karseius I. was often described as calm yet abtitious. He was often described as wearing the traditional Near Haradrim warlord facepaint as a symbol of his humble origin. Later decendants would not carry on this tradition.
Nothing is known about his early life. Karseius I. is first mentioned in a scroll from Black Numenorian Traders who describe him as warlord of the western Fertile Valley tribes. His influence grew rapidly and so he secured the title of Serpentlord to express his rule over all of Near Harad. With this he founded the Old Empire as well as his capital of Old Ain al-Harad. During his reign the city grew steadily but was never concidered more than a smaller city.
After Karseius I. found loyal bannermen like shamiir of Harandor and RookieNinjas I. he expressed intresst in the north. During the hight of his reign, he visited the back then weak Gondor and its back then King RedExtremeXD. Just with his sizeable army he was able to secure Harandor as a Near Haradrim Province and the river Poros as the northern border of his empire, as well as a colony in mainland Gondor known as Al-Anfal.
Karseius I. died relativly young. His mysterious death made the royal court of Near Harad very nervous which resulted in his sons being shipped away to safety in southern Harad. His legitimate successor and advisor, RookieNinjas I. took the throne after him, but he disappeared mysteriously which left the realm in chaos.
Although Karseius I. is an important figure for the Myths surrounding the Karssasid Dynasty and the Old Empire, his legacy is often overshadowed by the deeds of his later decendents.
During his reign he established the absolute monarchy as the traditionall political order of Near Harad which would dominate Near Harad over the course of history. Especially later rulers of the Karsassid dynastic family would follow this style of rulership.
There are several minor shrines spread across Near Harad to his memory. Most later rulers of Near Harad based their claims partly on his legacy.
Karseius I. was not an uncontroversal ruler. Especially near the end of his reign he is often blamed for the so called "Uncertain Period" during the Old Empire. After his death his successors were not strong enough to keep the peace in the large empire which resulted in chaos and many casulties during a state of de facto civil war.