Complex sentence: When Rourlaunch a surprise attack, Wentai releases Mulan from jail to let her go flee, but Mulan stays back to fight in the ongoing battle and eventually decapitates the Rouran general.
Split into simpler sentences: When Rourlaunch a surprise attack, Wentai releases Mulan from jail to let her go flee. But Mulan stays back to fight in the ongoing battle and eventually decapitates the Rouran general.