Something written by Blind Guardian. They did a song for a fantasy video game already. Guardian have lots of fantasy and medieval elements in their songs. Mostly inspired by Tolkien's novels or other fantasy books. Although they are a Metal band, they do have lots of ballads and slow songs, but also many very powerfull epic songs using orchestras and choirs. I think even if they do a ballad, this would be fine for non-Metal fans too.
Here is a song called "Valkyries" from their latest album. It starts with the sound of thunder...
Here is a midtempo song called "Nightfall" from the album "Nightfall in Middle-Earth"