Complex sentence: The occasion reminds the faithful of the core message behing Husayn's martyrdom: establishing justice and fighting injustice, no matter what its incarnation--a message that strongly influenced subsequent Shi'a uprisings against the perceived tyranny of Umayyad and Abbasid rule.
Split into simpler sentences: The occasion reminds the faithful of the core message behing Husayn's martyrdom: establishing justice and fighting injustice, no matter what its incarnation. A message that strongly influenced subsequent Shi'a uprisings against the perceived tyranny of Umayyad and Abbasid rule.