Complex sentence: In 2005, Viesturs became the first American, and 12th person overall, to summit all fourteen mountains over 8000 meters (collectively known as the eight-thousanders).
Split into simpler sentences: In 2005, Viesturs became the 12th person to summit all fourteen eight-thousander peaks. He is the first American, and 12th person overall, to summit all fourteen mountains over 8000 meters (collectively known as the eight-thousanders).