Death Pacts and Left-Hands Paths

Everyone is looking for shortcuts in life, but rarely do they find the kind they're looking for, and when they do it never turns out like they thought. But what if you were to accidentally fall into cahoots with an other-worldly creature who could provide those shortcuts and so much more? Of course, there's always a price attached to such favors, but killing gets easier the more you do it, and everything is great as long as the rewards outweigh the risk. That is until you find out this was never true and you've inadvertently set into motion something so horrible you lack the capacity to understand or accept it.