Their ascendancy had been preceded by a civil war among the Sunnis and Shi'ites known as the First Fitna. Hostilities only ceased when Ali's eldest son Hasan (who had been elected upon his father's death) made an agreement to abdicate in favour of the first Umayyad caliph, Muawiyah I, resulting in a period of relative calm and a hiatus in sectarian disagreements. This ended upon Muawiyah's death after twenty years of rule, when rather than following the previous tradition of electing/selecting a successor from among the pious community, he nominated his own son Yazid. This hereditary process of succession angered Hasan's younger brother Husayn, who publicly denounced the new caliph's legitimacy. Husayn and his family were eventually killed by Yazid's forces in 680 during the Battle of Karbala. This conflict marked the Second Fitna, as a result of which the Sunni-Shia schism became finalised.