Dominion was officially announced on the main website as the new capture-and-hold game mode on League of Legends on Thursday, August 4, 2011. Even though there was no information about this new mode before the announcemnt there was a large build-up for it on the Kalamanda storyline on the Journal of Justice Issues. During the development cycle, the mode was code-named Odin. 
The League of Legends community had no idea that such new map was being developed until the official announcement. However, its gameplay had been leaked the day before the announcement on the forums, and they had to be shut down as the news continued to spread.
The official announcement was accompanied by a video trailer and they are both here:
Many of you may have been wondering out loud just what we have in store for the future. Today we are pleased to give you your first look at League of Legends: Dominion, the largest game update in the history of League of Legends. This update will feature not only a brand new map, but also an entirely new game mode showcasing innovative gameplay never before attempted in the MOBA genre.
For quite some time, all of you have been clamoring for new battlefields upon which to test your skills against other summoners in League of Legends. We're pleased to announce that we've heard you, and that for over a year our Design Team has been working tirelessly to craft Dominion into a truly new and groundbreaking experience.
On the Crystal Scar players will battle for control of five capture points, holding them to damage their enemies' nexus. The game ends when one team had seized control long enough to drain the opposing team's nexus down to zero. Battles last around 20 minutes, and focus on high-intensity, player-versus-player combat. If you've been looking for a fast-paced, high-intensity competitive gaming experience, this is what you've been waiting for.
We're taking Dominion on the road during convention season this year, allowing you to get your hands on the new game mode at both Gamescom in Cologne, Germany and PAX Prime in Seattle, Washington! Stay tuned for more details as we draw closer to these exciting events.
As there was no information on how soon the mode would be available for the general public, the same post was updated with the following information:
Some of you have expressed interest in when Dominion will be available worldwide, and we wanted to respond to that directly. While we take a “when it’s ready” approach to our feature releases, you will be able to play Dominion in a nearly complete state at GamesCom (August 17–21) in Europe and at PAX Prime (August 26–28) in North America.
League of Legends: Dominion will make it into your daily play schedule shortly after PAX.
Keep an eye out on our website over the month of August for more information, video documentaries, trailers, live streams and gameplay previews.
During the following days, Riot Games released more information and a new website rearding the new mode.
The following statement and video were releaed on August 8. League of Legends Dominion Gameplay: The Legend Evolves
With League of Legends: Dominion just on the horizon, the goal was to really push League of Legends gameplay to the next level, providing a truly new and innovative experience for you. Whether you're a brand new summoner still leveling up or a grizzled veteran taking to the Field for yet another battle, we wanted League of Legends: Dominion to feel drastically different than our classic gameplay while still capturing those same elements that make League of Legends fun and exciting to play.
Focused on a capture and hold style of gameplay, League of Legends: Dominion promises short and intense game sessions filled with action-packed, back-and-forth battles. Unlike the more measured, strategic pace of classic gameplay, the frantic pace of this new game mode promises non-stop player vs. player combat as small groups of champions vie for control of dynamic map objectives that promise a whole new layer of team play and coordination. Matches are designed to be competitive right down to the bitter end, with late game comebacks serving as one of the hallmarks of this thrilling new game mode.
Here's a behind-the-scenes look into the design process of League of Legends: Dominion. Watch as members of our design team, and more of your favorite Riot personalities, explain their goals and objectives when constructing this exciting new game mode!
Art and Sound
We've put a lot of hard work into developing our upcoming game mode - League of Legends: Dominion . Our artists and sound designers have gone to the well to bring you a Field of Justice that really captures the fast-paced, dynamic gameplay of our new capture-and-hold game mode.
Unlike the mirrored design of Summoner's Rift, Crystal Scar features five capture points for players to capture and control, each with its own unique landmarks and style. We've also taken the overall environment detail to the next level, helping to create a greater sense of depth than we've ever attempted in the past. Combined with ambitious new particles, animations and brand new music that changes based on how your team is performing against your opponents, Crystal Scar is sure to provide a compelling new setting for the fast-paced, visceral combat in store for this coming update to League of Legends.
Here's an inside look at the construction of this brand new Field of Justice, and be sure to listen close for a sneak peek at the new music. Watch as some of Riot's gifted artists verbally illustrate what goes into building a new battlefield for the League of Legends: Dominion game mode!
League of Legends: Dominion is drawing closer every day, so it's time to reveal even more about this exciting new capture-and-hold style game mode. To that end, we've just released a major update to our League of Legends: Dominion Website !
In addition to new media and screenshots showcasing the fast-paced, champion focused combat, this update to the League of Legends: Dominion Website also features your first in-depth look at the Crystal Scar Field of Justice. By selecting locations on an interactive map, you'll be able to examine each of the battlefield's tactical objectives in great detail. You'll also find Dominion lore, along with information on various other gameplay innovations such as the scoring system, some neutral buffs, and the dynamic quest system.
Head on over to the official League of Legends: Dominion Website now for all the latest information on the next big update to League of Legends!
We launched Dominion more than four years ago as an alternative game mode designed to challenge players with fast-paced skirmishes, capture-and-hold gameplay, and different skills to master. Back then, we truly believed the mode would introduce new players to League of Legends and provide current players with an experience as captivating as Summoner’s Rift. As League continues to evolve, our vision for the core game becomes more and more refined, and, after years of learning, it’s clear today that alternative game modes work better in short cycles rather than as standalone queues.
We want to support game modes delivering consistently engaging and competitive experiences. Fewer than 0.5% of players actively play Dominion and we’re aware some number of even that small population is still bots. We haven’t been supporting Dominion, and we’ve trapped ourselves in a vicious cycle where the queue isn’t large enough to warrant major attention, but falls prey to bugs and periods of imbalance from the introduction of new champs and items (which then drives down the size of the queue, and so on).
In the absence of our attention, matchmaking and game quality in Dominion varies wildly based on daily engagement. Because of how matchmaking works, the longer someone looking to play Dominion waits in queue, the wider the pool of potential foes the matchmaker can pull from—with such a small population, Dominion games can match up widely divergent skill levels after long enough queue times. Unhealthy queues that force players to endure long waits with fluctuating match quality ultimately creates an overall poor experience—one we take responsibility for. We might have been able to break out of that downward spiral with more dedicated resources, but we chose Summoner’s Rift as the core League of Legends experience with its depth of gameplay, match pacing, and path to mastery. When it comes to the resources required to keep Dominion permanent and solve the inherent design problems and give it ongoing live balance support, we’ve consistently devoted them to Summoner’s Rift and related features because we felt they’d improve the overall League experience more.
For players who’ve made the Crystal Scar their home over the last four years, we’ll be taking a snapshot of the queue today just before this post goes live. Then we’ll give out an exclusive, never-to-be-reproduced Summoner Icon to anyone with over 100 wins in matchmade Dominion games (not including Co-op vs AI). You can expect the icon to land in your account within two weeks of this announcement! We know this doesn’t ease the sting of losing the mode you’ve spent thousands of hours in. But if you want to stick around and join us on the Rift, we feel you deserve a way to show off your domination.
We’ve learned a lot from Dominion and the costs associated with maintaining a fully separate game mode with balance changes, bugfixes, and item updates. We’re considering other ways to use the Dominion map (as a featured game mode, for example) but nothing is set in stone. We’re retiring Dominion to focus on our vision for League moving forward: we’ll concentrate on the core League game while supporting alternative experiences which deliver consistent, competitive gameplay with healthy queues and appropriate matchmaking (ARAM and featured game modes are good examples of this). We’re permanently disabling Dominion and removing the Crystal Scar from custom games on Monday, February 22..