According to "Yoshitsune Ki" (Biography of Yoshitsune), which was written during the Muromachi period, Tokiwa agreed to become Kiyomori's concubine on condition that he spare the lives of her children.
室町期に成立した『義経記』においては、常盤が清盛の妾になることを条件に子供達が助命されることとなったとある。