Starting on September 30th, Final Fantasy XIV will play host to the Moogle Treasure Trove, at least until the upcoming 5.1 patch launches. Leading up to the 5.1 launch, players can begin hunting down Tomestones of Mythology to exchange for rewards.
You can earn these Tomestones by completing quests and performing a variety of tasks, such as going through dungeons as a group - joining a dungeon in progress or using an undersized group won't yield the rewards. The type of dungeon itself will give you a different amount of Tomestones. You can check the full dungeon list and rewards, such as the new Mameshiba earrings, cosmetic gear, mounts and more, so be sure to check it out and see if anything you've been after is listed!
You can check out the full details on the official post on the FFXIV site. The event starts at 1am PDT on September 30th and runs until the 5.1 patch drops