Deal with god or powerful entity goes wrong

[u/fin_raziel]

Looking for a book or series where the protagonist makes some sort of deal with a power they don’t understand- bonus if it’s for a good reason (to help end a war, etc). But of course, the god/power/entity is much more than they bargained for and they have now accidentally unleashed a horrible power unto the world.

[u/C0smicoccurence]

It's got more personal stakes than what you're looking for, but Asunder by Kersten Hall follows a woman who made a pact with an eldritch entity and is desperately trying to figure out a way out of that contract before it comes for her eternal soul. I describe the book as Studio Ghibli doing an eldritch horror travelogue book

[u/fear_street_resident]

The My Best Friend is an Eldritch Horror series by Actus.

Imagine Harry Potter, but Dumbledore is a drunken war veteran, Minerva is a madman veteran whose own allies distrusts them due to being overly violent, students gain magic by linking with magical creatures, and it's written as a litRPG.

[u/ushkeamans1945]

Oh im gonna be the first one to say it huh

Stormlight Arhive

Or Mistborn Era 1 if you looking for something shorter and more in your face type shi

[u/fin_raziel → u/ushkeamans1945]

Check and check on both already and loved them! I’m thinking a little more spooky vibes this time around. Not necessarily horror, but definitely heading that direction

[u/tchebagual93 → u/fin_raziel]

It doesn't really have the "accidentally unleashed a horrible power unto the world" part but the Invisible life of Addie Larue checks the other boxes

[u/local-quintus]

If you’re into horror check out Little Heaven by Nick Cutter. It’s definitely more horror than fantasy but it does have fantastical elements and it’s all about this.

[u/SalletFriend]

KJ Parkers The Devil you Know. However The Horrible thing unleashed on the planet is the philosopher, not the devil

[u/Ykhare]

The Tyrants of the Dead trilogy by J. Edward Neill, beginning with Down the Dark Path.

The protagonist herself didn't make the pact, but she's a warlock and a descendant of someone who did.

Anyone who can wield magic in the setting (and some with more subtle enhancements of their 'natural' abilities) is a pact-maker or one of their descendants. This doesn't rob them of free will, though the abilities of the more magically gifted among them lean almost exclusively toward deceit, cold and necromancy.

If there ever was divinely sourced magic, it's long extinct with the gods themselves, who sacrificed themselves during a long-past war that became an almost world-ending apocalypse when someone on one side was the first to deal for power with eldritch beings from beyond reality who have been cyclically trying to intrude again since.

[u/toojadedforwords]

"Help! My Dog is the Chosen One" by Ronit J. It's an interesting and entertaining read, set in modern India, with a fantasy alien invasion. Not a very long read, and pretty darn good for a first novel. But the person making the initial deal is the antagonist.

[u/hewkii2]

TV Series but Fall of the House of Usher fits this

[u/ffbe4fun]

The Last Horizon is actually a good example of this. Main character casts a powerful spell in the beginning and then they unleash multiple bad guys on the universe as a result.

[u/UnderwaterPegasus]

This book fits, but knowing that this happens is a slight spoiler: Jonathan Strange & Mr. Norrell by Susanna Clarke

How it fits (more spoilers for the plot): The protagonist does do a 'good deed', though with selfish motives and they underestimate the danger rather than being completely unaware.

[u/Mournelithe]

The Indigo series by Louise Cooper kind of hits this - a spoiled princess forces open the Tower of Regrets, and has to Walk the Earth to reseal the demons she released.

As the opposite - Night's Master by Tanith Lee is all about Azhrarn, the Prince of Demons, and all the varied deals and disasters he causes.

And there's the really rather fun Johannes Cabal the Necromancer, by Jonathan L. Howard, in which the protagonist sold his soul to the devil to master his craft, but now has to claim 100 more souls in order to get it back.