The prototype art, which was at one point made available on an Ensemble-linked online artist portfolio website, further confirms previous rumors that the studio was working on an MMO based on the Bungie-created sci-fi franchise.
Notable, rumors reported in Game Informer magazine in early 2006 claimed that Bungie and Ensemble were teaming up to make a Halo-themed MMO. And, although it was not clear that it was Halo-specific, websites such as 1UP did point out that Ensemble was hiring for an MMO project as early as April 2006.
The title seems to have been in development in 2006 and some of 2007 -- though it's by no means clear that it was the only MMO-related title in development at the studio at that time.
What is clear, however, is that Ensemble's Bruce Shelley mentioned in a June 2008 blog post that they "...set up three prototype teams out of the staff of a major project that we cancelled. After six months of very interesting work, we have now stopped two of those prototypes, with one getting more time to demonstrate the value of its concept."
Thus, it appears that the Halo MMO project was likely internally cancelled some time in mid-late 2007, without any formal announcement from Microsoft that it was ever in development. The below UI mockup and concept/level screenshots also date from the 2006-2007 period.
Gamasutra had a chance to ask Massively.com lead blogger Michael Zenke about the MMO-related elements in the prototype UI screenshot (click through for a higher-res version):
Massively's Zenke explained the following on the prototype UI (of which there were several posted): "The character pane shows a health bar and 'mana' -- or Psion, as is referred to in this title... The upper right icons are for basic character functions. The one on the far left would most likely be inventory, represented by the outline of a man.
Moving right, I'd guess that's a 'talent tree' screen for customizing your character, most likely a powers menu (a brain to represent the Psion?), a titles or achievements screen represented by the trophy, and a support ticket system represented by the chat bubble icon.
There are many obvious tips of the hat here to the World of Warcraft interface. Simplicity and streamlined play appear to be the object of the game, but it's interesting to see so many strange icons on the hotbar.
From the choice of abilities, it's easy to surmise that that the game would have been some sort of melee/magic brawler. The upper bar has some similarities to the WoW UI modification Titan Panel - a favorite amongst players."
Also included were a number of in-game level pictures, both concept art and in-level shots, and Gamasutra asked 'Louis Wu', webmaster of the unofficial Halo website Halo.bungie.org to interpret them.
Wu simply noted of this concept screenshot: "None of those [specific] characters or creatures were EVER included in a Halo game."
Of this shot and others in the same series that Gamasutra has not yet printed, Wu commented: "Clearly broken Halo Warthogs, dead marines, a color palette last seen in Halo PC multiplayer levels like 'Infinity'. The presence of an actual Halo in the background of the last shot is also a dead giveaway [as to the game setting]."
Of course, the cancellation of the Ensemble version of a Halo MMO doesn't necessarily preclude other Microsoft-sanctioned developers from working on Halo franchise titles.
And with Microsoft's Phil Spencer recently mentioning: "There are more than two or three teams building Halo things right now", it could be that a Halo title with similarly expansive online options is now being tried at another studio. If so, there has been no public information about it to date.
[UPDATE: The 'Gone Is Gone' weblog has posted a full Flickr gallery of the concept art and UI mockups for the canceled prototype, sourced from the same art portfolio as Gamasutra's original story.]