Complex sentence: Stevens was the first sitting U.S. ambassador to die in office since Arnold Lewis Raphel in Pakistan in 1988 (in a plane crash), and is the eighth U.S. Ambassador to be killed in the line of duty.
Split into simpler sentences: Stevens was the first sitting U.S. ambassador to die in office since the 1988 aircrash in Pakistan which killed Arnold Lewis Raphel. Stevens is the eighth U.S. Ambassador to be killed in the line of duty.