Ivory Brony has been working on a great original composition called the Sombra Suite. Apparently he started it before Season 3 actually aired, based on the preview clip that was leaked. In any case, it's a great piece of music. I think it really captures the ominous nature of Sombra as he's portrayed in the show.
Listen to the Sombra Suite here. You should also check out Ivory Brony's Soundcloud channel to see his other work!