Its the Ruby Dragon Dungeon, shamelessly stolen from the facebook page's faq and page
Metal Dragons: Purpose
Q: What are metallic dragons used for?
A: Metallic Dragons, including the Metal Dragon #176, Ruby Dragon #252, Sapphire Dragon #255, Emerald Dragon #258, and Gold Dragon #179, are primarily found through metal dragon events which occur on any given day. Ruby, Sapphire, and Emerald dragons are available through one dungeon each week (see 'Metal Dragons: Event Timing'). Metallic dragons come in three forms, including the basic form, the 'high' form (e.g. High Gold Dragon), and the 'king' form (e.g. King Gold Dragon). Metal dragons are used to provide experience to other creatures and are, generally, best used to provide experience to same-element creatures. Basic metal dragons only provide about 1000 or 1500 experience. If the technical dungeons are available, basic Ruby, Sapphire, and Emerald dragons are best used in combination with Pengdra to get greater experience increases. Gold metallic dragons can also be sold for a generous compensation (3,000, 30,000, or 300,000) but, aside from selling basic gold dragons, it is generally recommended to use them for experience as well. Note that dark metal dragons and gold metal dragons may be worth saving for dark or light element creatures as these creatures do not enjoy experience bonuses from like-element Pengdra. Metal dragons are easy to defeat and, by far, the best way to level your team earlier in the game. They should not be overlooked.
Metal Dragons: Event Timing
Information:
http://bit.ly/WEtVcs (EN) ||
http://bit.ly/VrcX29 (JP)
Q: When do metal dragon events take place?
A: Each day will bring an event for a given metal dragon. Ruby, Sapphire, and Emerald dragons are available through one event each week. Metal dragons (dark element) are generally available on other days, except for approximately once a month when a gold dragon event will take place. Players are separated into five groups. You can determine what group you belong to by checking the third digit of your Player ID. (A: 0,5) (B: 1,6) (C:2,7) (D:3,8) (E:4,9). Each day, each group will have access to approximately three dragon events, starting as soon as around 8:00 AM PST and ending later in the evening before midnight. You can locate a metal dragon event by checking special dungeons each hour. Once you have identified the hour of a metal dragon event, you can return five hours later (events start on the hour) and the event will repeat. This Facebook group also kindly provides metal dragon schedules each day. Check the pinned topics for the group (it will generally appear if you click on the group name in your web browser) to see exactly when the metal dragons will be available for your group and what metal dragon will be available. Dungeon information: Metal Dragons (5 Stages, 15 stamina); Sapphire/Emerald/Ruby Dragons (5 Stages, 20 Stamina), Gold Dragons (7 Stages, 20 Stamina).
[US] Ruby Dragon Schedule (20 Stamina) 1/2/2013 PST
Check your 3rd digit on the left of your ID as it classifies which group you belong to
(A: 0,5) (B: 1,6) (C:2,7) (D:3,8) (E:4,9)
A: 8:00 13:00 18:00
B: 9:00 14:00 19:00
C: 10:00 15:00 20:00
D: 11:00 16:00 21:00
E: 12:00 17:00 22:00
Edit: Times are PST, which makes the later times awkward for GMT