The Seeker: The Dark is Rising:

Painfully generic-dumbed-down-fantasy-action trying to channel better film franchises instead of the atmospheric, mythopoetic and lyrical children’s book it is technically based on. Bonus points for the open contempt people involved had for the source material, both in how they treated it and what they said about it.

BAD. Bad bad bad!! They completely changed Will's character. In the books he does get frustrated sometimes, but is mostly kind and patient and really makes you believe that he is both an ancient being and an 11 year old boy and in the movie they changed it so he's like really mad and angsty and just the total opposite of his actual character!! Absolute butchery. And they cut the Wild Hunt! And changed a bunch of other plot stuff and it overall just sucked.

where do I even begin. they made the main character American for no reason (this is perhaps the most egregious change), aged him up to 14, and added a straight romance subplot. they were so indecisive during production that they CUT AN ENTIRE MAIN CHARACTER in the time between the trailer and the actual film release. they completely fucked up all of Will’s family stuff. in pursuit of “relatability” they got rid of everything that makes the book good and put in THE most generic, poorly written, poorly acted (except Christopher Eccleston, who did okay), and poorly produced garbage. it was in theaters for like…less than a week; we were supposed to see it for my birthday but it was already gone. it doesn’t even have, like, half-decent special effects. it is an insult.

Seventh Son (originally The Spook's Apprentice):

The book is a dark fantasy book for children, with fantastical elements that ring true to folklore but still have their own distinct flair, really well written horror centered on the fact that the protagonist is still just a boy, and a very strong theme that the spook opposes the forces of darkness and evil with knowledge and wits rather than heroics. The series as a whole has a lot of digs at established powers, both the Church or the nobles. It also has a mentor that is showed as imperfect from the get-go because of sexist tropes, and a protagonist who criticizes it even at the beginning when he's just a boy. The movie makes the protagonist an adult, turns the spook from an old wise man who practices using a stick (peasant weapon!) to fight and throwing silver chains to immobilize supernatural creatures, into the SURVIVOR of an ORDER OF KNIGHTS, and generally removes all the slow creeping horror to turn it into generic shitty fantasy movie #493, with bad acting on top. And it bombed so bad it probably RUINED the Spook's series chances to ever get a good adaptation, damnit!

Mentor gets changed from being part of a long line of people with a cool fantasy job to a discount Templar, cool methods of handling monsters got reduced to just killing them, awesome assassin grandma changed to Default Movie Witch, and more! It's pretty much not the story anymore.

It takes pretty much nothing that I liked from the books with the initial premise and some names being pretty much the only things kept intact.

The plot and world were completely changed. Most of the changes make for a much more generic and, in my opinion, worse story. Spooks in the books are seventh sons of seventh sons who are trained to fight creatures of the dark because seventh sons of seventh sons are able to sense the supernatural. They are hired for jobs by ordinary people, usually exorcising ghosts or capturing creatures like boggarts and witches. In the movie, the Spooks are also seventh sons of seventh sons, but for some reason, they are an order of knights. In the books lot of people think Spooks are charlatans until they come face to face with a ghost or creature of the dark because the world is usually pretty ordinary and peaceful. The setting is inspired by Lancashire and the world feels pretty unique. The setting in the movie is Generic CGI Fantasy Land where everyone has American accents and big CGI monsters are everywhere.

The protagonist Tom and Alice, his friend and later love interest, were aged up like in the Percy Jackson movies. In the books, Tom starts out as a 12-year-old and grows up as the series continues. He starts out very uncertain and wants to stay on his family farm. He only becomes a Spook because he knows he has to get a job to ease the financial burden on his family. He grows into his role as a Spook's apprentice over time. Alice is a pretty interesting character in the books, she is naturally very talented as a witch but struggles with the darker side of her powers over the course of the series. In the movie, they became incredibly generic Hero and Love Interest characters with pretty much no personality and instantly fall in love.

Mother Malkin was a first-book villain, who was mostly only such a threat because Tom was an extremely inexperienced 12-year-old at this point who accidentally released her and didn't realise how dangerous she was. She was monstrous and decrepit and fed on blood. In the movie she is turned into a beautiful seductress and a world-ending threat who can turn into a dragon. They also made her the Spook's lover for some reason.

Tom's mother is a really cool character in the books who pushes him to become a Spook despite him being reluctant and plays a larger role in the later books. In the movie, Tom is the one who wants to be a Spook and his mother doesn't want him to.

I think the character who got it worst was the Spook himself, John Gregory. In the books, he is a former priest at the end of his career as a Spook, he is incredibly uptight and disciplined to an extent that other Spooks find over the top. For example, he has a rule of fasting before a battle that he makes his apprentices follow and he only kills as a last resort. He's tough but usually pretty fair to Tom and explains things to him so he doesn't get hurt. In the movie he is pretty much the complete opposite, he's a drunkard who spends the whole movie making getting angry and fighting and doesn't seem very interested in teaching Tom how to be a Spook. Also, the accent that Jeff Bridges uses is terrible.