Aston Fisk serves the Mercian crown as part of a sect of assassins, known only as the Inquisitors. Abducted from birth and trained extensively in the arts of subterfuge and interrogation, he is selected to "eliminate" the leader of a well known Heretical group - the Orphites- before they can open a gate to then Black Realm and unleash the swarms of the undead upon Human kind. As he nears his target, he is flung through the portal into a world starkly contrasting to his own, along with a majority of the Orphites, who quickly decide that this new world they've stumbled upon should be destroyed as well.

Corporal Mason Gray and his partner, Staff Sergeant Matt Barker, are abandoned in the Ukrainian wilderness after failing a mission to assassinate a terrorist cell leader. Giving up, he accepts help from a mysterious spectre, feeling himself being transported somewhere else.
Once in Equestria, he discovers a plague, emanating from a dark pit in the earth, that threatens to eradicate pony-kind, just as it did to the dominant race before even Discord, and it needs to be stopped at the source.