Complex sentence: Eventually, Maria died as a result of the birth of their eleventh child and Ferdinand had to assume regency of Portugal (1853-1855) because his son King Peter V was only 16 years old.
Split into simpler sentences: Eventually, Maria died as a result of the birth of their eleventh child. Ferdinand's reign ended, but he had to assume regency of Portugal (1853-1855) because his son King Peter V was only 16 years old.