Complex sentence: At WWE's SummerSlam pay-per-view event on August 17,2008, Armstrong refereed the Hell in a Cell match between Edge and the Undertaker and has since moved to the SmackDown brand becoming its senior official.
Split into simpler sentences: At WWE's SummerSlam pay-per-view event on August 17,2008, Armstrong refereed the Hell in a Cell match between Edge and the Undertaker. He then moved to the SmackDown brand becoming its senior official.