Medea was an ancient witch. Jason found her at caves of his homeland on Iolcus. She was a fomer blacktongue and her powers was sometimes uncontrollable but she knew about that. Jason felt like he is going to need her in his journey so he took her on board of Argos. She used her magic to not allow her to leave her room in safety of the ship because of her temporary failuers of her will.
When the blacktongue appeared in steer room a killed Argo she knew about that but she couldn't do anything because she was prevent to do anything.
Back on Delphi with all desceants of gods she made Jason and two other argonauts (by player's choice) portal to Tartarus and allowed them safe return.