Theramore's Fall

Remix: This is available again for a short while, until August 19th, 2024, if you complete the Theramore's Fall scenario on a lvl 35 Remix: Pandaria character, then you will be awarded this achievement and the associated Toy. Note: you must be EXACTLY lvl 35 when the scenario completes, you can start it at 34 and ding mid run just fine, but if you hit 36+ before you complete it you will not receive this achievement.

Edit: This lvl 85 version is now confirmed to be available for only 1 week, Sept. 17-24, 2012.

Likely A limited time only Achievement (Pre-MoP event), and could possibly not be available to Pandaren (unless someone completes this then changes races).

Possibly Rewards [item=89205] for Horde.

Edit: It is now confirmed that there will be a lvl 90 version of this scenario, complete with a 10pt achievement.
However, since this is a lvl 85 FoS, the rewards and achievement are still likely to be limited time only.

Also, Theramore in the outside world will look normal to you until you complete either version of these achievements (85 or 90). Once earned, Theramore will be phased to its destroyed state.

Upon the release of 5.X, Theramore will be in its destroyed phase for everyone even if you never completed the scenario, and the quests for Dustwallow Marsh will be updated accordingly. (Though that might not have much impact on Horde questing in the zone).

This is from the 2012 Mists of Pandaria Launch Event
(Horde: Theramore's Fall / Alliance: Theramore's Fall)

The launch event awards [item=89196] / [item=89205] (depending on faction) to all players who took part in the event and did the full questline associated with it. Once MoP (Mists of Pandaria) launches, this FoS and the special event reward from it ([item=89196] / [item=89205]) will forever be unavailable.

If you wish to return to the pre-destruction phase of Theramore:

Speak to Zidormi just outside of Theramore at the far end of the long western bridge, midway on the Dustwallow Marsh map between Theramore Isle and Direhorn Post.

She offers the chance to "visit" Theramore before the destruction in the Theramore's Fall scenario (to assist players who still need to finish quests or just want to use the town's services). This phase change persists through logout. She will also return players to the "present day" (post-destruction phase).