Complex sentence: Mandukhai managed to keep Dayan Khan in power as a Chingis Khan's descendant and defeated the Oirats, actions which have contributed to the legends which formed about her life.
Split into simpler sentences: Mandukhai managed to keep Dayan Khan in power as a descendant of Ghengis Khan, and she defeated the Oirats. Both feats have contributed to the legends which formed about her life.