Going down the easy way
I know I mentioned a different story coming first in FEA, but I'm having a bit of a block with it, sorry.
I own nothing but my own Ideas
Someone father trusted the most? Who else would it be but mother? The woman with Grima's mark, etched clearly across her hand. The tactician.
You need her now, need her strategy to win the war, to become her mother for a brief time.
But this time you will not be left an orphan!
You can play your part. There is a part of you that wants this, who wants a mother. It will serve for now.
As long as you never forget who she will become.
You had almost believed you had managed to change things, but you know now it was a lie. You have seen her turn, even if it was for a moment, and now it is time to end her lies, before she hurts him.
You wont use Falchion, its too recognisable. Instead you creep close enough to sink a knife into her neck. There is a pleading in her eyes when she sees you, and confusion. You hesitate, wondering if this is the right thing to do, then shake yourself. Such thoughts do not belong here. Not after what she put you through.
"This is for father." You whisper. "For the future." Before she can respond, you plunge the knife into her throat.
Your body burns, and you go to find a river to cool it, and wash away the blood and guilt.
Ever since mother died, you have been staying close to father. You don't believe Grima has given up, and don't trust anyone else to watch his back.
The Shepards believe that Robins assassination had come because she was a queen. A foreign threat to the nation of Ylisse. They had been on the right track, but barking up the wrong tree.
You even manage to convince father to take you with him to face Validar at dragons table. No one he trusts will let him down this time!
Father strikes the final blow, and turns to face you.
"We've done it!" You think excitedly. But that's the moment it all goes wrong. Without warning, you body turns on you, sword thrusting through his body.
You drop it horrified.
"What have I done?" You ask. But no one is there to tell you, lost as you are in the shadow of the Dragon.
Later your brother, cousin, and aunt will come. Trembling, your Morgan picks up a weight he never asked for. Falchion. He hands it to Owain.
"I'm not one for blades."
There is a lot he isn't saying. "I don't know how I exist, I was never born here" Is probably forefront in his mind.
He cries, mourning his family torn apart by the battle of dragon gods. He doesn't mention Naga's whispers telling him that he would not have made a good Grima vassal, that he had the wrong temperament for it, because truthfully he would have given anything to become the vassal, if it meant everyone else was still alive. He's not a charismatic ruler, or a driven leader, or the brilliant tactician his mother was, not in his mind at least.
You know he is thinking of this, because you had been his confidant, sister and best friend once, and he had shared his fears and hopes with you.
You sigh walking away. Better get out of here before you make his day worse.
"Whoever fights monsters should see to it that he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you. " Friedrich Nietzsche
I always though that Lucina would be the more interesting Grima vassal of Robins kids, and this story is one such scenario. Theoretically, killing Robin would kill Grima, but Chrom also has dragon blood, in my mind making his and Robins kids viable candidates, and the problem of killing Grima is not as simple as it appears on the surface, like in the game, as Nagas ritual only impedes it.
I'm not sure that I should have added that bit about Morgan and Family at the end, but I wanted to see how he would take it.