*
Why did she think Sariel could solve all of her problems? She brought attention to the conflict with Eterna to the sentinels too. Each time, it ended in failure.
It can't be. Was she trapped in the past once more? No matter how many times she does it, she can't ever get used to it. Then this... It makes her first trial with Sariel. Just like all the other times, it ended in a sea of fire. No matter how much of a resistance the magical girls put up, the magical girls either get eliminated, or Eterna retaliates and drowns humanity in flames. This time too, Sariel couldn't do anything. No one could. There's no surprise now, it's clear she can travel through time in some form.
In a manner that evaded Eterna's notice. In a way that evades the universe's notice. One that completely reconciles with the present no matter what happens. It sounds nearly invisible, and the only way this sort of method could be achieved in reality is by simply not travelling in time at all. One could call it clairvoyance, or prophesising. She sees entirely different branches of the timeline, how their futures play out, decides what should be done to avoid those futures, all in her mirrors.
Transcendental technique - wonderland. Only a limited amount of information could be brought back, and only a limited amount could be brought 'in' too. After all, the method for activation is for her to enter the mirror and experience the 'future' for herself. Most of the time, she'll forget that she's in a mirror due to this limitation. Not knowing she resides in an inconsequential future will guarantee that she worked harder, at least that was how she dismissed it.
"I see, so it was you. Right, if it was an instantaneous expansion of information, it would indeed be near impossible for the real me to track.
Then, could you simulate her? The being at the boundary of the universe. " (Eterna)
"... No. If you're right that she can break the world just by existing, her presence alone would shatter the mirror. I couldn't simulate her without breaking my mirror, by then the shards won't form a coherent image. " (Alice)
"I see... I'm not even real. I'm a future you simulated. Since when...?" (Sariel)
"Since-" (Alice)
"Oh, don't take this the wrong way. We are simulations, yes. But we are in a mirror, we are also a reflection. If the real me agrees, we can use it as reference and instantly copy the state of this world to the real world." (Eterna)
A false reflection need only look to reality's approval, if the 'real' side conforms to the false image, then is it really false? The state of this world will be copied wholesale, so Alice's and Sariel's memories won't incur any gaps. Someone as all powerful as Eterna, she didn't even think they would constitute much of a threat when they regain their memories. And a being that could make Eterna feel fear.
"It's unfortunate, but I won't let that happen." (Sariel)
Billions would perish. Every electrical grid and gas pipe exploded, highways had their support collapse. The residential zone is a sea of fire, less than 10% of people would survive such a thing. The 9 billion+ lives ride on her shoulders.
"How do you proposed that? The Alice in here and the Alice in the real world is different, by her own mechanism when this Alice dies, it transmits memories to the real one. She can't defeat me either." (Eterna)
"A world distorted beyond the flow of fate, sinking into the murky unknown. This is a 'failed future' and it shall remain as such.
Oblivion Magia, reverie across stars." (Sariel)
"Cancel." (Eterna)
"Oblivion Magia!!!" (Sariel)
"Cancel!!" (Eterna)
Sparing no effort, she crushes Sariel's skull as she negates her Oblivion Magia.
"Parallel processing, {Swap} completed" (Sariel)
Her voice echoes from an unoccupied plot of air. A thin line stretches from the ground to the sky before splitting into radial patterns like a web, then it shattered. An illusion set up by Alice. Eterna was fighting a reflection of Sariel, and the reflection gave the same signals as the original. Sariel hid within the reflection, so there was no contradiction. For a single lethal attack, the reflection can take on the burden but would undo her safety net. However, Sariel hadn't been idling in vain the entire time.
She was calculating. Calculating coordinates of the nearest blackhole, and bringing it here. She spawned it on top of Eterna. She had done so before in a fight with Lastia, though on a smaller scale. She turned it from night to day, bearing light and pressurised air down on her opponent. The world around them grows dimmer as the blackhole ravenously consumed all light. Eterna stood calmly at its side, her eyes trained on Sariel as if looking at prey she could finally snatch at.
"T-this... What did you think would happen? " (Eterna)
With a single snap, she not only withstood the blackhole without flinching, she sent it back to where belonged. No matter what Sariel did, if it was a preparatory move, Eterna would undo it and demolish her efforts. What she needed to do was to kill her in one move with an attack more potent than a blackhole, something that could destroy the universe all at once. That is what she lacked most, a direct attack, much less one with that sort of potency.
If she was capable of that, there wouldn't have been so much worries on her mind when dealing with Eterna's prior harassment. Let's say she miraculously dealt with Eterna here? Wouldn't the real one need to be dealt with too?
No, she wouldn't falter. Real or fake, there's no such value in distinction. She can only deal with what she's been dealt, pushing her best efforts forward. Alice's authority over the mirror won't help her defeat the reflection of a deity, but she can jam this Eterna's attempts of sending signals to the real Eterna. Otherwise the Alice in the real world would be in trouble. 9 billion lives lay on her shoulder, her mission hasn't changed one bit.
So what if it's a losing battle? So what if she's destined to suffer defeat? Nothing changes her drive. She's a fake, a reflection. Something void of substance. Her entire existence was meant to fizzle out when light left the mirror. However, just for now she's able to deliver actions with meaning. She can do something for reality by eliminating this threat here and now. Or she'll die trying.
Because she's a magical girl.