Ixion is the soul reaper and harbringer of death. He only appears during times a lot of people die to reap their souls.
He appear during the Crystal War due to it being that, a war, which means a lot of people will and/or have died. During the previous expansions, not a lot of people died, which means he had no reason to show up. Some people foresaw this, and went out to the battlefield in an attempt to catch a glimpse of him and catch him. Thus, he occasionally appears in a zone where Campaign is currently taking place, leaving prints and clues behind him, not very much unlike Odin. Rarely you might be able to catch a sight of him, and by defeating him you can use his powers for your own.
There, I fixed the storyline for SE. And we suddenly gave SMN's the ability to get a rare avatar that's probably rather useless, but still have one ability which is actually decent and get put to good use. At the same time we implemented yet another HNM to camp, which rarely appears and can not be camped due to the random factor of it and the amount of zones he can appear in, like Tiamat only harder to camp and easier to defeat, thus forcing people to invest time in getting something that only slightly (if at all) increases their efficiency.