Fairy Tail Biggest Mysteries 2: God Slayers and God Slaying Magic [discussion]

Now this probably is, if not the, biggest mystery in the entire series, questioned by many of us fans of this series. Due to the focus on Dragons and even the Demons, we haven't had much focus on God Slayer Magic.

God Slayer Magic is a Slayer Magic exactly like Dragon Slayer Magic used by Dragon Slayers, ones we know like Natsu, Wendy and Gajeel, and Devil Slayer Magic as we know is used by Gray and was used by Silver. Except that this is meant to Kill God's, literally, Zancrow, Sherria and Orga who are all God Slayers confirm this. What's weird is we've only seen Three God Slayers in the whole series, two of them only being worthwhile as Orga was essentially beaten up and one shotted like cannon fodder.

Zancrow even describes this Magic as the way of "Hunting Gods" and Sherria even said against Chronos that God Slayer Magic was made to kill evil Gods. The Gods in Fairy Tail are already a strange subject to discuss because we see so little of them in favour of the Dragons, Dragon Kings and Dragon Gods. So when Sherria says this could it be that there was a war that came before even the Dragon King Festival? A war where select humans were chosen to slay Evil Gods. Or a religious cult that raised children to become God Slayers? Sort of like how the Dragon Slayers were raised as kids by Dragons to slay Acnologia eventually.

God Slayer Magic in general feels incredibly powerful as Natsu couldn't eat Zancrows Fire initially, he had to figure out an alternative way of doing it which almost killed him, something we have never seen Natsu having to do in the entire series. We have seen him consume Etherion and Laxus' lightning but after one attempt with Laxus he could do it perfectly and retained that power, with Etherion he was knocked out but went right into a weaker Dragon Force. Zancrows flames had almost killed Natsu on several occasions and even incapacitated him after usage. Meanwhile, Zancrow could eat Natsus fire like nothing, he ate it just how Natsu would consume a random flame. This in itself proves one thing, God Slayer Magic initially in concept not depending on the users power level is stronger than Dragon Slayer Magic, or at least has more capability and hax to it, and that makes sense given the premise that it is meant to slay Gods. It even has Breath attack variants with "Bellow" instead of "Roar" or "Rage", and even a God Slayer Secret Art which we see Sherria use, something Zancrow and Orga haven't used, and we have still yet to see Gray use a Devil Slayer Secret Art.

What I don't understand is, this very interesting mysterious magic is introduced into the series and yet we've seen God's die very easily. Natsu one shotted a War God with his Fire Dragon King Destruction Fist, in base. This has always been something very interesting to me about Slayer Magic because when it comes to Devil's and God's, it's clear they can be hurt by Humans or Dragon Slayers, in Tartaros case their curses proved troublesome and they were very powerful, and yet a Dragon cannot be harmed at all by a human unless they have Dragon Slayer Magic or at least a Dragon Slayer seal, which I believe is a very unfair advantage as there are humans in the FTverse currently that Narratively in strength should be above the level of at least fodder Dragons and yet they wouldn't be able to harm them because of Dragon Scales. And yet God's appear to be capable of dying by the hands of anyone in their echelon of power. I've seen a few theories regarding this magic suggesting Zeref created it and that's why it's Black Because it was meant to slay Ankhseram, which could be the case but I doubt it as it probably would have been mentioned by Zeref if he sought to kill Ankhseram rather than reverse the world back.

One thing about this magic that has always interested me is the fact that you can learn this magic from a book. Sherria learned it from a book and Zancrow says he learned it from Hades, presumably the same way Ultear learned Arc of Time which naturally would have both come from Books. Whilst Dragon Slayer Magic can be self taught as we've seen with Elefseria, it was always taught by Dragons mostly or required a special Lacrima or even consuming a piece of a Dragon to be used which I believe is a more believable and better way for such a powerful magic to be given to someone. Given that this Magic was even conceived of and uses God-like elements I would assume that the Gods gave mortals this magic through teachings in case they were ever faced by an evil God.

A further question is, what is the downside of this? Lost magic always has a side effect, especially Slayer Magic, Dragon Slayer Magic transforms one into a Dragon through overuse, Devil Slayer Magic causes one to become a Devil and lose their sanity, so what does God Slayer Magic do to a person? Does it turn them into a God? As in Transcendence, perhaps Transcendence like Zerefs Fairy Heart or even Acnologias Time Rift powers? If Zancrow overused this magic would he undergo "Deification"? Becoming a Demi God of Fire, as Zeref was able to completely control the concepts of Space and Time with Fairy Heart would Zancrow completely control the concept of Fire? Or is it something a little more grounded, like losing your sanity or eventually being erased from existence like Zordio. Just what exactly is a God in Fairy Tail? They're clearly not a race, they are divine beings, is there a level or Transcendence like in Bleach where beings with Transcendent Reiatsu become Gods? As we know even something like Takeover God Soul makes you recognised as a God in the FTverse given Sherrias effectiveness of her God Slayer Magic on Chronos.