From Halopedia, the Halo wiki
It was formed by ex-Ensemble employees, including former-CEO Tony Goodman and is comprised of 45 former Ensemble employees, after the shut down of their studios by Microsoft in September 2008. They are responsible for the maintaining of Halo Wars, as well as the creation of new content.
Their first game to be released is a collaboration with Microsoft entitled Age of Empires Online, a free-to-play Games for Windows Live game. It is a continuation of Ensemble Studios's Age of Empires series.