Complex sentence: Rana rule over Nepal started with Kot Massacre, which was conducted in Kot, near Hanuman Dhoka Durbar, where most of the high ranking officials of Nepal were massacred by Jung Bahadur Kunwar and his supporters.
Split into simpler sentences: Rana rule over Nepal started with the Kot Massacre, which occurred near Hanuman Dhoka Durbar. During this massacre, most of Nepal's high ranking officials were massacred by Jung Bahadur Kunwar and his supporters.