Originally, the game supported 3d tv's, so this sky was built in 3d instead of using matte paintings like we usually do. You get a better sense of depth, but it also drags down the performance more than a matte would do.
This thing was a challenge since there was so many other things running at the same time. The Hecaton character itself is larger than any of the titans we had in previous games.
The cameras show 360 so we cut some parts of the sky as soon as the cameras don't show that way. I used mattes for the clouds and for the art close up, I used a mix of zBrush, World Machine and old school Photoshop.