Complex sentence: He came into office when the previous president, Cyprien Ntaryamira, was killed in a plane crash (an assassination in which President Juvénal Habyarimana was also killed), and he left office when he was deposed by Pierre Buyoya in a military coup of 1996.
Split into simpler sentences: He came into office when the previous president, Cyprien Ntaryamira, was killed in a plane crash (an assassination in which the Rwandan president Juvénal Habyarimana was also killed). Ntibantunganya left office when he was deposed by Pierre Buyoya in a military coup of 1996.