Characters who have already completed the event once will not be able to clear it again, of course (although they can feel free to participate in the event FATEs along the way). Clearing the full quest line opens up a set of cosmetic armor based on Iroha’s outfit from FFXI and FFXIV. Just be sure to take part in the event before it goes away on November 29th if you’d like to get the appropriate garb; the crossover starts on Friday, November 10th.
Final Fantasy XI, and Final Fantasy XIV wants to celebrate that milestone. But how? Why, with a trip back to the Final Fantasy XI celebration event, naturally. It’s good for old fans of the game, for players who missed the celebration the first time, or just people who can’t get enough of seeing a character from one game cross over into another game.
Source: Official Site