I don't think it even exists. Apart from catching a couple legendaries and doing the Looker Missions, there really wasn't any real postgame content in Pokemon X. I seriously hope there is a substantial area to explore for the next games because having nowhere new to go makes the existing regions become repetitive and stale. I don't really get why it's so hard to implement anyways. Just make a couple more cities filled with ultra-tough trainers. Create a small subplot to go along with all the areas with the story culminating with a boss fight against a trainer with nearly all LV 100s. Make the postgame very challenging because the harder something is, the more fun it gets (unless it's just unfair/broken).