Complex sentence: At this point Namri Songtsen (Namri Löntsän) was the leader of a clan which prevailed over all his neighboring clans, one by one, and he gained control of all the area around what is now Lhasa by 630, when he was assassinated.
Split into simpler sentences: At this point Namri Songtsen (also known as Namri Löntsän) was the leader of a clan which one by one prevailed over all his neighboring clans. He gained control of all the area around what is now Lhasa by 630, then he was assassinated.