Halo MMO was to have been "new focus" for Ensemble
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Ensemble's Halo MMO, codenamed both Titan and Orion - was to be the developer's "new focus" beyond Halo Wars, according to this Gamasutra article on the firm's closure.
A new campus was even being built to house development for the game, but work was was halted when Microsoft finally reach a decision to close the studio.
Apparently, work started on the game as early as 1998, "when a designer named Ian Fischer started pitching a sci-fi MMO. He loved the genre, and felt it was under-served," according to that piece.
The game first came to light in September last year, when art was leaked online.