User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5341032-20130921030100/@comment-3146930-20130921040704

Part of Percy's request at the end of The Last Olympian was for all the peaceful Titans be freed, which he made with Calypso in mind. If the gods help up their end of the bargain (which was made on the River Styx), then Calypso should be free to leave her island if she wishes.