The departure was Tweeted by his teammate Patrice Desilets.
When Assassin's Creed creator Patrice Desilets was fired by Ubisoft, Desilets' parting words were that he'd "fight vigorously for my rights, my team, and my game." Ubisoft would seem to be taking that statement seriously. Jean-Francois Boivin, another former Assassin's Creed alumnus who jumped ship to THQ at the same time Desilets did, only to be brought back to Ubisoft when THQ went under, has also been fired by Ubisoft.
Boivin's departure was Tweeted by Desilets. Boivin was a senior producer at THQ before the company's collapse, working on AAA IP. When Desilets was terminated, Ubisoft chose to obfuscate the issue, saying that "the good faith discussions between Patrice and Ubisoft aimed at aligning Patrice's and the studio's visions have been inconclusive" and therefore Desilets "left the studio." Presumably Boivin's vision also didn't fit the Ubisoft credo.
What this means for THQ title 1666, which Desilets and possibly also Boivin were working on, is anyone's guess. It seems unlikely, at this point, that it will ever see the light of day; or, if it somehow struggles out of Ubisoft development hell, that it will in any way resemble Desilets' original conception.
Source: Game Informer