What actually happened at Karbala

explain the bolded part plz if u cud. I am part of an org that has organizd quite a few progs so all nature and I seem to have missed out on sumthing.

btw if u want to talk abt commercialism then Hajj has turned into a business like no other. So has Eid, Eid ul Adha, Milad, Naats-nasheeds etc.