Auron was killed by Yunalesca but he was simply unsent. There are plenty of unsents in the world of Spira and Auron is no different. He didn't want to "die" and refused to let go. He wanted to stay alive, only so he could carry out the promise he had made to Jecht in regards to Tidus. I don't remember any part of the game saying Auron was revived, he just simply died and didn't go to the Farplane.