Complex sentence: In 1813, while a prince, Mianning also played a vital role in repelling and killing White Lotus invaders that stormed the Forbidden City which earned Mianning important merits in securing his claim for the throne.
Split into simpler sentences: In 1813, while a prince, Mianning also played a vital role in repelling and killing White Lotus invaders who stormed the Forbidden City. This action earned Mianning important merits in securing his claim for the throne.