Igneel did not fight END. Zeref said "Igneel was unable to kill the demon" That reason being *SPOILERS* because END is Natsu and Igneel had gotten fatherly feelings toward Natsu. Acnologia did a dragon soul extraction spell which killed all Dragons the only they were able to survive was hiding inside a Dragon Slayers body but once they got out of that body Acnologias spell would then kill them. While they were in the bodies they also had another mission and that mission was to stop the Dragon slayers from turning into dragons so they would become like Acnologia and before they died they made antibodies so once they left there Dragon Slayers body the antibodies would still prevent them from turning into dragons.