Complex sentence: The race was marred by the death of Jochen Rindt, who died during the qualifying session on September 5 and went on to be Formula One's only as to date posthumous World Champion.
Split into simpler sentences: The race was marred by the death of Jochen Rindt, who died during the qualifying session on September 5. He went on to be Formula One's first (and to this day, only) posthumous World Champion.