The events at Karbala are among the better-attested episodes of early Islamic history in the sense that both traditions agree on the outline: Husayn refused allegiance to Yazid, set out for Kufa on the invitation of its people, was intercepted, was denied water, and was killed with some seventy companions and family members, his surviving relatives taken captive to Damascus.
What the event became is a distinct question. Shia devotion has developed the narrative into something with a structure of its own, a majlis in which the story is recited to produce weeping, an established literature of elegy in Arabic, Persian, Urdu and other languages, and the taziya passion drama of Iran, which is among the very few developed theatrical traditions in the Islamic world.
The theology attached to it is substantial. Husayn's death is read as a deliberate, foreknown sacrifice against tyranny, and it has been mobilised politically with great force, most visibly in the Iranian revolution of 1979, where the slogan that every day is Ashura and every land is Karbala framed the shah as Yazid.
Sunni tradition honours Husayn as the Prophet's grandson and a martyr, and regards the killing as a crime. It does not attach the commemorative apparatus, and some Sunni scholars have criticised the mourning practices as innovation.
Pilgrimage to Karbala for the fortieth day after Ashura draws crowds reported in the tens of millions, which makes it among the largest annual gatherings of people anywhere on earth.