Calypso is the Greek goddess of the sea and the nymph who seduced Odysseus, imprisoning him on her island for seven years. While her initial actions may have had negative overtones, she eventually did release him and graciously helped him in preparing for his journey home. She also aided Odysseus by helping him build his ship and providing him with favorable winds for sailing.
Tia Dalma/Calypso is desperate for Pirate Lords to assemble and release her from her human bond. To do this, she resurrects Hector Barbossa to retrieve Jack Sparrow from Davy Jones' Locker and convince the Pirate Lords that the East India Trading Company is threatening them all.
After rescuing Jack, Calypso is convinced that the Pirate Lords will agree to meet at Shipwreck Cove. However, during Tia's incantation to undo the spell binding her to mortal flesh, Calypso learns of Davy Jones' betrayal.
At the conclusion of Dead Men Tell No Tales, when Captain Will Turner begins to turn into a sea creature like Davy Jones, it's because Calypso has summoned the stone crabs to fetch him the Black Pearl. When the crabs appear, they're bound to carry the Pearl and their Master's commands without question.
After being freed from her mortal form, Calypso grows to enormous proportions and shatters into thousands of crabs that whirl into a mass and create a massive whirlpool and storm in the sea. When the crabs eventually revert to their human form, they find themselves in a small South Pacific island that is exposed to radiation from nuclear weapons testing, similar to the ones conducted on Bikini Atoll by the United States in the 1950s.
