The initial concept for the game's introduction was to feature a big battle frozen in time with the camera sweeping elegantly through it, slowly revealing more as it did so. Achieving this took longer than originally expected, and involved writing our own custom tone mapping and 2d animation software.
Another key element is the music - we chose Mozart's Lacrimosa as we felt its epic scale and dark themes fitted the mood we were aiming for in the intro. It was chosen relatively early in the process so we were able to carefully synchronise the music and animation so they really gelled.
In the end, the animation took two months to create and an additional month to render. However, we are very pleased with the end result, a frame of which you can see in this screenshot.