Sao culture in the Logone valley and around Lake Chad as early as the 5th century. Little is known about the Sao, except that they were copper and ironworkers and, legend says, fearsome giants. Legends of later peoples claimed that the Sao were descendants of the Hyksos who arrived in the area in several waves. Another theory makes them the original inhabitants of the Chad basin, traditionally an oasis to the north of the basin. They may have been of Nilotic origin. Even the span of their civilisation is in dispute, with various estimates putting their rise at some point between the 5th and 8th century and their fall between the 8th and 15th century. The prevailing opinion among scholars, however, dates them to no later than the 10th century. At this time, eastern invaders entered the Chad Basin and conquered them. The Sao likely disappeared through intermarriage with this and other groups. Many of the current ethnic groups in the Far North claim descent from the Sao. The Kanem Empire, originated in the 9th century AD to the northeast of Lake Chad, and from there expanded in the region. Islam reached Kanem early, and the rulers converted in the 11th century. Kanem-Bornu strengthened as an Islamic stronghold, and the population converted as well. Embassies were traded with North African states, and trade increased. The rulers launched a series of conquests, culminating in Dunama Dabbalemi's (r. 1221–1259) expansion south as far south as the Adamawa Plateau. At this point, Kanem and its successor Bornu were active participants in trans-Saharan trade. Slaves in particular were frequently traded in exchange for horses and salt. The Kanem-Bornu Empire lasted until 1893, when the Sudanese warlord Rabih az-Zubayr overthrew it. Other groups in the territory also formed kingdoms, such as that of the Kotoko. They were eventually split, as the northern Kotoko became a vassal state of the Kanem Empire, while the southerners paid tribute but remained mostly independent with their capital at Logone-Birni. This part of the kingdom would eventually be Islamicised as well. The Mandarawa settled the Mandara Mountains in the 15th century. They coalesced into state, called Mandara. Over the next hundred years, they fought wars of expansion against their neighbours, eventually capturing Dulo, which would become their capital. Civil war erupted after this conquest, and an appeal was made to the ruler of Bornu in 1614 to settle it. The