Complex sentence: If a ship's cat fell or was thrown overboard, it was thought that it would summon a terrible storm to sink the ship and that if the ship was able to survive, it would be cursed with nine years of bad luck.
Split into simpler sentences: If a ship's cat fell or was thrown overboard, it would summon a terrible storm to sink the ship. If the ship survived, it would be cursed with nine years of bad luck.