Complex sentence: Calypso explains that he had not fulfilled his task of bringing the people who had died at sea to the other world; he had thereby corrupted his purpose and his mind and therefore was not able to break the curse of having to stay at sea.
Split into simpler sentences: Calypso explains that he had not fulfilled his task of bringing the people who had died at sea to the other world. He had thus corrupted his purpose and his mind and therefore was not able to break the curse of having to stay at sea.