The event is available to players level 30 and above. You can enter the event through any Adventure Assistant or Khatru Pup, which can be found in any major city. The event takes place underwater, and anyone who dies in this event is teleported back to the starting point, a small safe zone platform, with no Exp loss or item drops.
Official Guide: Temple of the Dragon - Guide
You can exit the Dragon Temple by taking the "Leave the Palace" quest from the Dragon Temple Events Agent, located on the starting platform. If you have any Moonwater Keys, you should sell them to the Events Agent or to another player before leaving, since they vanish when you exit the instance.
Which is used for the New Oracles Lv.2
The Automated Event will occure in the following Schedule: