NOLINK

RACE TO MOUNT DOOM was a league produce released by Decipher around the time of the Two Towers era. It consisted of a set of "meta-sites" that participants would travel down; while "at" a meta-site, you would receive a randomized set of game text for that position which would alter the game in some way or form. Typically, lower-ranked meta-sites would have easier (even beneficial!) text, while the higher up the chain you went, the harder and harder the text would get. It thus acted like a handicap system of sorts, keeping successful players on their toes as they went up the ladder.
You can read all about the original RTMD on the wiki here.
RTMD has now been implemented as a constructed league type on Gemp! All 64 original modifiers--some from the original Towers release, some from the King edition, and some from the Lord of the Rings Online TCG platform--have all been implemented. Modifiers have had their text tweaked to match more modern templating, and 1 modifier had to be altered to work with Gemp's architecture, but besides that all the original Decipher modifiers are available on Gemp.
On the Events tab, you can join a RTMD league just like you would any other, by clicking "See details" and then "Join league". You can view the meta-sites for that league in the details view, and also see which level you are at (indicated by the yellow highlight). This will also show how to advance--it will say whether you advance with wins or points. In the screenshot below, you "Advance every 4 points", so after 2 wins, 4 losses, or a combination of the two.
You can click or right-click on the meta-site to get a full-sized preview.
In the deckbuilder, the Race to Mount Doom league will show in the bottom of the target format dropdown:
Once selected, this will automatically attach the RTMD meta-site for your current level in your deck for you:
You should do this for any decks you intend to use for a RTMD league. Some modifiers alter the deckbuilding options you have available, such as changing the maximum number of copies of a card from 4 to 6! You will need to have this target format selected to properly validate your deck.
Premiere Leagues
To kick off Race to Mount Doom, there are 3 leagues available to play all through the next 22 days! Decipher recommended a set of 9 starter modifiers, which have been put into the Towers Throwback League. Two more sets of modifiers have been hand-picked for Decipher Fellowship and PC-Movie, so go try them out!