Hundreds of thousands of black-clad Muslim pilgrims thronged to the revered shrine city of Karbala in Iraq on Tuesday to observe a major religious ritual. The annual pilgrimage, known as Arbaeen, marks the end of a 40-day mourning period for the martyrdom of Imam Hussein (AS), the grandson of the Prophet Mohammed (S), in the seventh century. At the time, Imam Hussein had revolted against the Damascus-based second Umayyad caliph, Yazid bin Muawiyah, moving from Madinah to outside Karbala in modern-day Iraq, where a battle took place.