After the death of Mu'di Karab Ya'fir in around 521 CE, a Himyarite Jewish warlord named Yousef Asar Yathar rose to power. His honorary title Yathar means "to avenge". Yemenite Christians, aided by Aksum and Byzantium, systematically persecuted Jews and burned down several synagogues across the land. Yousef avenged his people with great cruelty.^[42] He marched toward the port city of Mocha killing 14,000 and capturing 11,000.^[40] Then he settled a camp in Bab-el-Mandeb to prevent aid flowing from Aksum. At the same time, Yousef sent an army under the command of another Jewish warlord, Sharahil Yaqbul, to Najran. Sharahil had reinforcements from the Bedouins of the Kindah and Madh'hij tribes, eventually wiping out the Christian community in Najran.^[43] Yousef or Dhu Nuwas (The one with sidelocks) as known in Arabic literature, believed that Christians in Yemen were a fifth column.^[44] Christian sources portray Dhu Nuwas (Yousef Asar) as a Jewish zealot, while Islamic traditions say that he threw 20,000 Christians into pits filled with flaming oil.^[42] This history, however, is shrouded in legend.^[35] Dhu Nuwas left two inscriptions, neither of them making any reference to fiery pits. Byzantium had to act or lose all credibility as protector of eastern Christianity. It is reported that Byzantium Emperor Justin I sent a letter to the Aksumite King Kaleb, pressuring him to "attack the abominable Hebrew".^[40] A tripartite military alliance of Byzantine, Aksumite and Arab Christians successfully defeated Yousef around 525–527 CE and a Christian client king was installed on the Himyarite throne.^[45]

Ruins of The Great Dam of Marib

Esimiphaios was a local Christian lord, mentioned in an inscription celebrating the burning of an ancient Sabaean palace in Marib to build a church on its ruins.^[46] Three new churches were built in Najran alone.^[46] Many tribes did not recognize Esimiphaios's authority. Esimiphaios was displaced in 531 by a warrior named Abraha, who refused to leave Yemen and declared himself an independent king of Himyar. Emperor Justinian I sent an embassy to Yemen. He wanted the officially Christian Himyarites to use their influence on the tribes in inner Arabia to launch military operations against Persia. Justinian I bestowed the dignity of king upon the Arab sheikhs of Kindah and Ghassan in central and north Arabia.^[47] From early on, Roman and Byzantine policy was to develop close links with the powers of the coast of the Red Sea. They were successful in converting Aksum and influencing their culture. The results with regard to Yemen were rather disappointing.^[47]

A Kendite prince called Yazid bin Kabshat rebelled against Abraha and his Arab Christian allies. A truce was reached once The Great Dam of Marib had suffered a breach.^[48] Abraha died around 555–565 CE; no reliable sources regarding his death are available. The Sasanid empire annexed Aden around 570. Under their rule, most of Yemen enjoyed great autonomy except for Aden and Sana'a. This era marked the collapse of ancient South Arabian civilization, since the greater part of the country was under several independent clans until the arrival of Islam in 630.^[49]