#Dark Magic being so tied to Fomortiis and giving to its users a bootleg copy of his abilities is telling enough

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@zeroabyss replied to your post “@zeroabyss replied to your post “You're not wrong...”:

I definitely say a father telling him he has to do it all on his own and that he can’t even rely on his friends or even their fellow nations, definitely pushed Lyon to take the path that he did. No one can handle something on their own, and so Lyon was tempted to use the one power their texts said had the power to avert disaster.

​You can see how this issue was more than likely a flaw when Lyon talks about him to the twins. About how his father took on the burdens all on his throne and barely, if ever rested. The man shouldered everything that it likely played into his worsening health and death, and his final words being to tell Lyon he can’t rely on help from others concerning a natural disaster and that the other countries would leave any Grado refugees to die at the border to protect themselves

Vigarde pushing all of the country's hopes and aspirations on his unprepared son was his mistake, sure, but...

Between Vigarde telling him to deal with Grado's fate alone because relying on Renais is impossible, and Vigarde/Father Mc Gregor/The World telling him not to free Satan to use his powers because Satan nearly destroyed the continent centuries ago, if Lyon had to pick one command to disobey, I suppose he could have picked the "I'm still going to ask for Renais' help" instead of "I'm going to use Fomortiis' powers".

Vigarde might have steered him in the wrong direction - Lyon already playing with Fomortiis' powers (to save a child! and then to predict tempests and save more lives!) was his own thing.

It's SS in a nutshell, good intentions alone cannot justify the worst means used to satisfy/reach them.

Lyon lamented and complained about his perceived weakness, feeling he would never compare to the twins nor would be the kind of ruler Grado will need to save itself... so he doomed Magvel, wanting to be the strong and kind person he wished to be.

I think this is the flashback where it's more or less spelled out :

Lyon: “Mm… My research isn’t complete yet, but there’s no mistaking the power the Sacred Stones contain. And the Stone of Grado seems especially responsive to my dark magic. I suppose it’s because it contains both sacred and demonic properties. Regardless, if I can just press my studies a little harder… Well, I think I might be able to use it to heal my father’s illness. If that works, there’s no telling how many other people I can help as well.” Eirika: “Do you think this is wise? The power contained within the stones is beyond our understanding… My father tells me that the stones possess a power not to be trifled with.” Lyon: “Uh-huh… Father MacGregor is also against it. That’s why they still won’t give me direct access to the Stone of Grado. For the time being, I’ve made do with the radiant energies surrounding it. They’re nothing compared to the raw power of the stone itself, but… If I can show Father MacGregor some real progress, perhaps one day…”

(...)

Lyon: “Thanks, both of you. Hearing those words from you means a lot to me. Actually–and this hasn’t been made public yet–but… I’ve already saved someone using the knowledge I’ve gleaned so far.” Ephraim: “You have?” Lyon: “Uh-huh… A while back, a fire ravaged Serafew, and a little girl got trapped in the flames. Her burns were terrible: not even healing staves were able to cure her injuries. But just a sliver of the stone’s power restored her life and healed her wounds. We saved that girl’s life, Ephraim! Oh, if you could have seen the tears of joy in her mother’s eyes!

Fado, Ismaire and I'm pretty sure Vigarde himself know it's not wise to triffle/use the SS powers... Father MacGregor is also against it, as the resident "holy man" around - and his words are important, since the Church of Magvel was founded after Latona, who fell herself (but managed to fend him off for some reason ???) to Fomortiis' possession! - but Lyon's reply is... Mockery? Or disinterest?

Father Mac Gregor wanted to prevent him from continuing on this path, and forbade him access to the Stone! But Lyon didn't care and still used what was... available, when he also knows the power he is using has demonic properties, and a will of its own!

Lyon's reasoning isn't "Father Mac Gregor is right maybe I should try to consider that I cannot "use" Fomortiis" but something like "maybe he doesn't believe me when I tell him this power can be used for good! I just have to continue working on it!" completely missing the point...

Or is he?

Sure, Lyon saved the Serafew child... but this hadn't made been public as of yet (2 years before the start of the game), like was this not enough to convince Father Mac Gregor that he could use those powers to do right and good things? Or Lyon himself realised he... "saved" someone who was already dead?

This flashback is even more interesting because we have this, the twins' failure :

Ephraim: “Using the Sacred Stones to study magic…” Lyon: “Ephraim, Eirika, what do you two think? Using the divine power of the Sacred Stones in this way… You don’t think it’s a very good idea, do you?” Ephraim: “…It’s just that I know nothing at all of magic. I would that there were some way I could help you, but I simply can’t. But, Lyon, I’ve seen how hard you’ve worked to help others. I know how much you want the power to make others happy. I know these things, and because I know you, I trust your intentions.” Lyon: “Ephraim…” Ephraim: “If you hope to use the power of the Sacred Stone for good, I trust you can. I’m behind you all the way.” Eirika: “I agree with Ephraim entirely. You spend every night in the library, studying cures for your father’s illness. The power of the Sacred Stones is too powerful to be used for personal gain. But you, Lyon… I think you’ll be fine. You’re the kindest person I know.”

It's a double failure on their part, firstly because they give empty reassurances to Lyon claiming to know him, how he wants to use this power because he has the right intentions, or how it's totally not used for his personal gains...

When part of Lyon's desire for power was to help people, sure, but also, to be worth someone in Ephraim and Eirika's eyes!

Secondly...

No, the power of the SS (especially the one housing Fomortiis!) cannot and should not be used, even for "good" or with "good intentions". There are lines that shouldn't be crossed, even if it means not being able to save people/not reach your dreams.

The Epilogue in Eirika's route is very explicit :

Eirika: “That would be nice… But first we must seal this away… This stone banished the Demon King. With such power… Do you suppose we could bring back Father and Lyon… That everyone who died in this dreadful war could be–“ Ephraim: “Eirika.” Eirika: “I know, I know. Such things would be– I cannot wish for such things. No matter how it hurts, or rather because it hurts, we must learn to accept sorrow. We must take it into our hearts and tame our grief…” Ephraim: “Eirika… Tell me, Sister… If Lyon were here with us, what do you think he would want?” Eirika: “What he would want?” Ephraim: “Yes. You see, I think I know. It was the reason for his studies. It’s why he wanted to know about the Sacred Stones–or rather the Fire Emblem. Let’s take Lyon’s dreams and fulfill them in his stead. We can do that, can’t we? Without relying on any mystic power?” Eirika: “You’re right, Brother, we can. Let us fulfill his dreams. As Lyon wanted, as we all want… So we may live in peace and joy.”

Sure this epilogue sort of sucks because Eirika has to be explained things by Eph out of anyone (when we know how he takes the news in his route!), but the point still stands, tempted with the power to achieve the impossible... the heroes refuse and accept to move forward and fulfill their dreams (make the impossible possible?) without relying on mystic powers, especially if it comes from Fomortiis himself!