Complex sentence: During the return journey, while aboard the ship, Columbus wrote a letter summarizing his experiences and announcing his discovery of the "islands of the Indies", that he would send to the Catholic monarchs Ferdinand II of Aragon and Isabella of Castile upon his arrival.
Split into simpler sentences: During the return journey, while aboard the ship, Columbus wrote a letter reporting the results of his voyage and announcing his discovery of the "islands of the Indies". He sent that letter to the Catholic monarchs Ferdinand II of Aragon and Isabella of Castile upon his arrival.