Age Of Empires creator Ensemble Studios owned by Microsoft was announced to close up, a big secret was discovered by Gamasutra that a Halo MMO was in development at the studio according to many prototype UI and level screenshots unearthed from an Ensemble-linked online artist portfolio website.
This Halo MMO seems to have been developed between 2006 and 2007:
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.
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