Hiya, stumbled upon this post while searching MC-related stuff. I figured its too late to give any advice seeing that the last post was almost 2 years ago, so this post is more to offer another perspective in case anyone else stumbles upon this post.
Anyway, as someone who's played modpacks with three of those mods in the past and have experienced them to different degrees, I disagree with Alistair_Wonderland's advice. Choosing to imagine that the magic from these mods is "the power of God" doesn't actually change anything. The "magic" of these mods are still what they are no matter what you imagine them to be. If a magic mod is satanic and you pretend it's holy, it'd be no different from telling yourself that a rotten apple is actually fresh: you'd just be deluding yourself, and you'll still get food poisoning if you eat it.
So in short, be honest with yourself and view these mods for what they are. If they seem like a bad thing and make you uncomfortable, it'd be better to go with your gut and leave them alone than to tell yourself they're something that they're not just to ease your mind.
.... now for the tough part. Are these mods bad? Will they lead to spiritual harm or bring in demons? To be frank, I can't really say yes or no, since there's no way for me to know for sure. I can give you my own personal opinions and experiences with the mods though. (And while I'm at it, I'll address some of the points Alistair_Wonderland made and why I disagree).
The only magic mod I really messed around with was Ars Magica 2. I only experienced Witchery and Blood Magic indirectly through my friends. Back then, I chose to avoid those mods since they seemed wrong and made me uncomfortable. I knew witchcraft was bad and Witchery included certain concepts that were really iffy, like voodoo dolls (IIRC that was a Witchery feature). Blood Magic also made me uncomfortable and seemed really iffy, though I didn't have an exact reason why at the time due to a lack of knowledge (maybe the Holy Spirit was warning me?).
On a side note, my view on fictional magic systems is that if they're based on actual occult practices or just generally promote immoral behavior, then they should be avoided. Also, if a fictional magic system gives you a bad feeling, like you know something's wrong with it (which could be the Holy Spirit telling you something), then it's better to avoid it than to stick with it.
Anyways, now for the mods themselves:
Ars Magica 2 doesn't
seem to have anything bad in it... but that's only from my limited knowledge. I'm not sure if Ars Magica 2 is based on any actual occult practices or if it's all made-up fantasy magic. I can't remember if I had any bad vibes about it either. My uncertainty concerning Ars Magica 2 was actually how I found this thread, 'cause I was wondering if other Christians used the mod and if they thought anything about it.
Ironically, the mod I used the most out of the magic mods is the one I feel least convicted about. Not sure if that's a good thing or a bad thing.... Bottom line, remember to talk to God about these kinds of things if you're uncertain.
Witchery, if it's based on actual witchcraft, is a big no-no. It doesn't matter if it's in a virtual environment, practicing witchcraft in any way is just asking for trouble. Even if it doesn't bring anything in, it could hurt you spiritually by making you complacent with it.
And on Alistair_Wonderland's point on vampires and the angel of death... nothing personal, but that's a really big stretch. I'm not sure how the whole thing works, but angels are God's servants, and from what I understand, an angel of death is simply an angel who God sends to kill people (like the one spoken of in Exodus).
By contrast, vampires are unholy creatures of the night who literally suck the life out of people and are harmed by holy things, whether it be blessed bread from a church or an image of Jesus Christ. Pretending that a vampire is an angel of death is a disservice to angels.
Blood Magic. Not sure if Blood Magic is based directly on existing stuff, but there's a deep, blood-drenched history when it comes to pagan/occult religions and blood sacrifices. Spilling blood in exchange for power is just plain iffy.
And on Alistair_Wonderland's point on there being sacrifices in the Old Testament, he's completely omitting the context behind these sacrifices, and how Israelite sacrifices differed from every other religion's sacrifices.
The Israelites sacrificed sheep, as it was instructed by God and it represented the sacrifice Jesus would later make on the cross for our sins. Which is why Jesus is often referred to as the Lamb, 'cause sacrificial lamb and all that. Jesus's death also fulfilled the very purpose of these sacrifices, so they're no longer necessary.
Any other ritualistic blood sacrifice would be a pagan/occult one. The history and spiritual significance of spilling blood runs deep. It isn't something to be taken lightly.
Anyway, Alistair_Wonderland makes a point about God's powers being supernatural in nature, beyond our understanding and capable of amazing feats. God also worked through the prophets to do amazing things like prophesize and conduct miracles. These are all true. But I feel like he blurs the lines a bit, and that's a dangerous thing. A lot of what people think of as "magic" is pagan and occult practices. These practices are false in nature, but they are dangerous because they appeal to demons, who are also capable of supernatural feats in this world. They are nothing compared to the power of God, and they fear the very name of Jesus... but the moment you let them in by conducting occult/pagan practices, or by even being complacent with such practices, they have a level of dominion over you and can effect your life in negative ways. It's kind of like surrendering spiritual territory. The more you give, the more they can take.
I'm not saying this to frighten anyone, or to make them hyper-neurotic about the media they indulge in (speaking from personal experience). I'm just saying you should be wary of such things, because they
do matter and
can have an effect, and pretending they're something else to ease your conscience is a really bad idea.
Anyway, that's my (really long) take on it.