Ars Magica's supplements can actually do a fantastic job of this.
Counterintuitively, Ars Magica without the magi can support this feeling of desperate dread quite well.
When suggesting Ars, I must recommend against my academic-philosophical-mechanical playstyle. I treat the rules as a fascinating treatise on Aristotelian thought, and play accordingly. This, happily, is a choice. By removing the core "magic" component and using the Divine, Infernal, and Lords of Men suppliments, you have a world where magic certainly exists but is overshadowed by the Infernal and the blessings of the divine.
The benefit of using Mythic Europe is that a huge amount of setting information is available; look at the history section in any bookstore. At the same time, the myths and traditions of the various religions that have evolved from that era... have a lovely contradictory pastiche in modern minds. This makes predicting the capabilities and intentions of people in the game difficult.
From a pragmatic perspective, each of the books also includes fantastic research into that aspect of the setting. Devils are not merely mechanically defined, but explored. Their origins, motiviations, and reason for existing are all well discussed, with plenty of story hooks provided for the players.
By making players normal villagers (maybe with a hint of magical virtues or divine virtues... or infernal virtues) existing with a significant infernal presence nearby, families, and insufficient resources to just flee... it is quite possible to have a hierarchy of Devils opposing the players in ways that the modern paradigm just rejects: thereby increasing the disscoation and allowing fertile fields for Horror.
To amplify: "modern paradigm" vis a vis religion and beliefs, the faiths detailed in the Divine supplement are accurate to the 13th century. They provide a metaphysics and theology appropriate to their time period and concept-of-world. Unless one of your players is a scholar of historical theology, the concepts and world-experience of 13th century faiths, especially if played to the hilt, can be profoundly alienating and discomfiting.
The non-magic rules for Ars Magica are very simple and usable. And many of the supernatural powers can be handwaved, especially if the party has no one at a comparable power level. It is only through cunning, prayer, and sacrifice that the erstwhile heroes have a hope of succeeding, and there's nothing in the rules that presumes a "party" or even "roles."
Have the players roll up people at the "companion" level of power, don't worry about the magic system, and read up on the infernal, the divine, and the normal ways of life in Mythic Europe... Everything is set for a very very horrific game.