The Gates of Tartarus were opened in a climatic battle between two forces of demons, and before they were closed, countless things crawled forth from that pit into the world. Everywhere they went, they infested, dominated, infiltrated all aspects of life. A sickness soon crept over the lands, a shadow as unrelenting as the dark of night.

Their kingdom drawing closer and closer to poverty and ruin, the two princesses of Equestria send for aide. In time, they receive an answer. A hunter whose expertise in all things supernatural is legendary; a dragon, of all things, world-renowned for his deeds, honor and might.

The Slayer.

(Note: Spike is an anthropomorphic dragon, but the majority of the other characters are humanized/straight-up humans.)