Husayn ibn Ali, third Shia Imam and grandson of Muhammad (b. 626)
Al-Ḥusayn ibn ʿAlī ibn Abī Ṭālib (Arabic: الحسين بن علي بن أبي طالب; 10 January 626 – 10 October 680 CE), also known as Abu Abd Allah or Imam Husayn, was a grandson of the Islamic prophet Muhammad and a son of Ali ibn Abi Talib and Muhammad's daughter Fatimah, and a younger brother of Hasan ibn Ali. He is the third Shia Imam after his brother, Hasan, and before his son, Zayn al-Abidin. He is considered a member of the Ahl al-Bayt as well as the Ahl al-Kisa, and he was a participant in the event of Mubahala. Prophet Muhammad described Husayn and his brother, Hasan, as "the masters of the youth of Paradise."During the caliphate of Ali, Husayn accompanied him in wars. After the assassination of Ali, he obeyed his brother in recognizing Hasan-Muawiya treaty, in spite of being suggested to do otherwise. In the nine-year period between Hasan's abdication in AH 41 (660 AD) and his death in AH 49 (669 AD), Hasan and Husayn retreated to Medina, trying to keep aloof from political involvement for or against Mu'awiya. After the death of Hasan, when Iraqis turned to Husayn, concerning an uprising, Husayn instructed them to wait as long as Mu'awiya was alive due to Hasan's peace treaty with him.

Prior to his death, Mu'awiya appointed his son Yazid as his successor, contrary to the Hasan-Muawiya treaty.

When Mu'awiya died in 680, Yazid demanded that Husayn pledge allegiance to him. Husayn refused to do so. As a consequence, he left Medina, his hometown, to take refuge in Mecca in AH 60 (679 CE). There, the people of Kufa sent letters to him, invited him to Kufa and asked him to be their Imam and pledged their allegiance to him.

On Husayn's way to Kufa with a retinue of about 70 men, his caravan was intercepted by a 1,000-strong army of the caliph at some distance from Kufa. He was forced to head north and encamp in the plain of Karbala on 2 October, where a larger Umayyad army of 4,000 arrived soon afterwards. Negotiations failed after the Umayyad governor Ubayd Allah ibn Ziyad refused Husayn safe passage without submitting to his authority, a condition declined by Husayn. Battle ensued on 10 October during which Husayn was killed along with most of his relatives and companions, while his surviving family members were taken prisoner. The battle was followed by the Second Fitna, during which the Iraqis organized two separate campaigns to avenge the death of Husayn; the first one by the Tawwabin and the other one by Mukhtar al-Thaqafi and his supporters.

The Battle of Karbala galvanized the development of the pro-Alid party (Shi'at Ali) into a unique religious sect with its own rituals and collective memory. It has a central place in the Shi'a history, tradition, and theology, and has frequently been recounted in Shi'a literature. For the Shi'a, Husayn's suffering and death became a symbol of sacrifice in the struggle for right against wrong, and for justice and truth against injustice and falsehood. It also provides the members of the Shi'a faith with a catalog of heroic norms. The battle is commemorated during an annual ten-day period during the Islamic month of Muharram by many Muslims especially Shi'a, culminating on tenth day of the month, known as the Day of Ashura. On this day, Shi'a Muslims mourn, hold public processions, organize religious gathering, beat their chests and in some cases self-flagellate. Sunni Muslims likewise regard the incident as a historical tragedy; Husayn and his companions are widely regarded as martyrs by both Sunni and Shi'a Muslims.