Achilles was an ancient hero best known for his immortality, with only one place where he could be hurt and that was his heel.
In Rise of the Argonauts he was the undefeated champion of the Mycenean Arena. Jason was sentenced to trial in the Arena on Mycenea and his final fight was against Achilles. At the end of the fight when both are badly wounded but still keep on fighting, King Lycomedes ends it with a result of draw and claims Jason innocent.
When sad king Lycomedes declared another tournament and winner of that tournament would be a new king of Mycenea Achilles didn't sign up for the tournament as expected. Jason persuaded Achilles to take part in the tournament but he refused. Jason sign up for the tournament instead of him but at the final fight he appeard and help Jason to kill the Docon and blacktongues. At this time Patroclus (arena owner) tried to kill king Lycomedes during the fight, but Lycomedes dodged him and smashed his head with his mace.
At the end Achilles asked Jason if he can join them at their jouney and Jason gladly accepted him.