He left Valve some time around 2005, after he had finished work on Lost Coast, and then went to work at Arkane Studios, where he did work on the Source title “Dark Messiah of Might and Magic”; and the legendary, yet on hold “The Crossing”; and, perhaps, even the infamous HL2: Episode Four, which we shed some light on in this post; as well as this one. He appears to have left Arkane at some point after that.
But now Antonov and Arkane are back together, working on “Dishonored”, a first person action title, produced and published by Bethesda, scheduled for early to mid-2012. They are joined by Harvey Smith, one of the game designers behind System Shock, Deus Ex and Thief. And the game does sound a lot like them. It also looks quite a bit like HL2 itself, despite its steampunk-inspired aesthetics.
If you want to familiarize yourself with Antonov’s artwork, feel free to check out his page on the Combine OverWiki, and his official site. Kotaku has even done a Fine Art story for his art (they’ve also done a Fine Art for TF2’s art), though all of the art in their Valve Fine Art posts can be found online.
In any case, we wish Antonov, Smith, and the rest of Arkane Studios best of luck with this project and their future endeavours. It looks very promising. You can find more information at Game Informer’s Dishonored information hub.