Look up on youtube Two Steps From Hell Ashes. Two Steps make music for movies/trailers and promotional events. Ashes is really good with Grim Dawn, as it makes a little more edgy and fun.
Or if you want some monster slaying music about the end of the world zombies and monster horror check out Demon Hunter. Their music was featured in a popular pc game (zombie fps) Killing Floor 2. The songs are specifically about killing monsters and zombies.
Collapsing,
Not I,
My Destiny
Life War
Someone to Hate,
infected,
and more!