Excerpts from Books and Wikipedia
"On October 10, 680, Husayn ibn Ali, the grandson of the Prophet Muhammad, was killed in a battle that saw the deaths of male members of the Prophet's family and the capture of women and children. . . . The violent death of Husayn is one of the most significant events in fueling the spread of Shi'a Islam, and its anniversary is observed as day of public mourning. To Shi'a Muslims, the tomb in Karbala of the decapitated martyr, Husayn ibn Ali, is one of the holiest places on earth." [Furtado: 1001 Days]