Complex sentence: Between 1997 and December 2008, at least 37 eventing riders died as a result of injuries incurred while competing in the cross-country phase of eventing at national or international level or at Pony Club, and of these, 18 riders died in the period 2006--2008.
Split into simpler sentences: Between 1997 and December 2008, at least 37 eventing riders died as a result of injuries incurred while competing in the cross-country phase of eventing at national or international level or at Pony Club. Of these, 18 riders died in the period 2006--2008.