Complex sentence: Abu Nidal was the founder of a militant Palestinian group known as the Abu Nidal Organization, who at the height of his power in the 1970s and 1980s was widely regarded as the most ruthless of the Palestinian political leaders.
Split into simpler sentences: Abu Nidal was the founder of a militant Palestinian group known as the Abu Nidal Organization. At the height of his power in the 1970s and 1980s, he was widely regarded as the most dangerous and ruthless of the Palestinian political leaders.