As you've requested, Princess, I've looked into how Weiss may have used Merciful Flash in the past. I hope this is sufficient to help you with your plans.
Thank you Sunset. I truly appreciate the work you've done for me. If I can master this spell, one of ponykind's sole incurable maladies will be no longer.
Though rare, a damaged horn is the source of no small amount of grief for a unicorn, one which I and the rest of medical science had believed to be beyond our ability to mend.
Are alicorns not at risk too?
No, dear Sunset. The qualities of Pegasus and Earth Pony magic within us means that not only are our horns flexible rather than brittle, but that they can also recover naturally from damage.
Otherwise, I doubt myself or Luna would have horns as healthy as our own, after the years we have spent arrayed against Equestria's enemies.
Entry Number Five Hundred, Year Two-hundred and Ninety.
I've recently grown beyond the point where the Divine Words serve any purpose. I no longer need artificial lenses and mirrors in order to control my Unicorn Light when casting spells. But even so, it isn't enough. Various assassins have taken to targeting my horn, first and foremost, believing a clean, powerful blow would be sufficient to disable me. Even though I can use Merciful Flash to regrow parts of my body freely, I've grown sick of my foes mocking me, believing me to have vulnerabilities.
I can grow Unicorn Horn using Merciful Flash. So why should I bother to wait for my horn to be shattered to do so? There is no suitable answer to that question. So I'm going to do exactly that. I've grown thirty thin fingerbones made from artificial unicorn horn, and have replaced my cloned limbs' fingerbones with them.
I had expected this to trigger some sort of metamorphosis to my magical state, but I haven't changed at all. Maybe there's nothing left to change, or maybe this is merely a placebo, and I've always had the capability to cast unicorn light from my fingers? It's irrelevant now, I suppose. This is the new me.
The addition of Unicorn Horn to my hands has inadvertently allowed me to cast far more complex magic, not because my ability to multi-cast has increased, but because I'm no longer crowding myself when attempting to cast a spell that requires me to use my horn. Now, instead of having to layer two, or even three spells onto my horn, I can simply layer them over my fingers or hands, giving me more room to work with the magic.
The use of Unicorn Horn in my bones has more purposes than merely enhancing my magic. There exist rumors that claim the bones of an Alicorn are made entirely of the same substance as their horn, hence their name. Which would make sense, considering that a Unicorn's horn is very nearly indestructible. Of course, my own horn has been rendered a bit scrappy near the tip from stress testing this fact by ramming adamantium walls to see who wins, but It hasn't interfered with spellcasting, so I'm ignoring it for now, due to the immense cost of growing unicorn horn via magic.
Some unknown part of myself has also decreed that I use very specific amounts of artificial horn in my new fingers. Each of my hands now has roughly the same amount of synthetic unicorn horn in it as a full actual unicorn's horn.
That would make seven in total. An auspicious number, to be sure. Seven Chaos Emeralds, Seven Human Souls, Seven Pure Hearts.
Seven Unicorn Horns.
In spite of this, however, one thing still eludes me. Even after hundreds of years, I'm still not strong enough to cast a spell I've been devising ever since that day. A spell capable of killing Sombra outright, no matter what.
I call this spell the Smoke Devouring Hydra.
I have seven foci with which to cast it, but I still can only cast four spells simultaneously, leaving three of my arms useless and untapped. I feel it, somewhere inside of me, some part of me that I have failed to eradicate in return for power. But for the life of me, I can't find it. Despite my efforts, despite my desire, I'm still unable to summon the Smoke Devouring Hydra, without the force of will and emotional power needed to cast seven spells at once.
It might seem like an arbitrary distinction. Four Hydras, Seven Hydras, what's the difference? It isn't the number of hydras that is the problem. It is what it represents. Weakness. I've all but isolated myself from the Shadow Realm's public sector, excluding my management of it and it's defenses, I've delegated much of the lawmaking to my council, I have cut myself off from any friends I still have, and yet I still can't reach that last font of dark power inside of me.
I'm unsure how much further I can take my studies into the dark arts. What sacred things are left for me to defile for power? What lines could I possibly have left to cross in the pursuit of vengeance?
Even after all of this, my heart still has some tangled shreds keeping it away from the event horizon.
This is... bizarre to me. Weiss only attributes the name of "Hydra" to his perfected attacks. Does he simply decide before creating them whether or not he will master them until they are without flaw? Some form of precognition? Or maybe he was simply utterly confident in his ability to master this "Anti-Sombra" technique.
Why does he name them after Hydras?
Well, usually, it is because they use all of his arms. Such as the Extra Thick Hydra that his Kelpie Gear had originally used.
Ahh, listen to me, reminiscing. He originally created that Gear to fight dragons, you know. It's ironic that in improving it, he decided to escalate to becoming one.
Then why did he use it against you? I don't intend any disrespect, but wouldn't it have made more sense for him to come up with some kind of countermeasure that was designed from the ground up to fight you?
Well, Sunset, you were there. He chose that form because he had wanted to lose that fight. If we tussle again, though, I've been working on a few trifles of my own, to give a better showing.
Case in point, if I master this spell, my magical reserve and regeneration exceed Weiss's by an order of magnitude. I'll be able to produce a substantial amount of synthetic unicorn horn to play with, even outside of the obvious medical usage it will take part in.