something in arm makes a new touma with all memories

So what exactly had happened?
To understand that, we must change our viewpoint and move back in the timeline a bit.
“Ah, ahhh, ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!!!???”
The sound of something bursting open sounded on the thick ice covering the sea between
Great Britain and Ireland. This was just before the end of the war when everyone was
focused on the Queen Britannia as it sank along with Coronzon and Aleister.
But something else entirely had been happening then.
It was a much smaller and more personal issue than something influencing the fate of
nations or the entire world.
Particles of ice had blown up like a blizzard, so Index must not have been able to see what
happened even while she dangled from the Bunny Gray balloon.
Kamijou Touma’s right arm had exploded at the shoulder.
Nothing external had caused this.
In fact, it was blown away from within by a power it could not contain.
“––––––!!”
The pain was too much to bear.
He had no idea what he was shouting as he held his torn right shoulder with his other
hand, collapsed to the thick ice, and rolled around. The wallet and phone fell from his
pocket, but he did not have it in him to pick them up. He was too busy making sure he did
not bite his own tongue.
Even so, he could not bring himself to shut his eyes.
Something was happening in front of him.
(What the…?)
It was his right arm.
It was floating in that empty white space like it was defying gravity.
No, this was not all that strange. He had seen it several times before. He had just always
turned a blind eye because it had always worked out in his favor, so he could not start
calling it creepy as soon as it fell out of his control.
The end of his detached right shoulder twisted while dripping with red blood.
No.
There was a small clicking sound.
(What is that?)
Just as he wondered that, a strange artificial-looking triangular prism appeared about
fifteen centimeters away from the bloody end of the detached right shoulder. The sides
were divided into keyboard-like squares that were moving in and out all on their own.
There was no way he could ignore this.
That strange object really was there in front of him.
“Kazakir—no, this is different!” he shouted.
The triangular prism shined with a pale platinum light and he grimaced.
It happened silently.
The triangular prism gained a head and a right arm past the shoulder.
It all fit together perfectly and resulted in an identical boy standing there in front of him.
This went well beyond wondering where this other boy had come from or how he had
appeared here.
Was there no way to explain this using nothing more than biological science?
And the monster was wearing the exact same clothing that Kamijou was.
“What…?”
He could only just stare blankly up at him.
No, staring blankly did not qualify as choosing an action.
With his right hand gone, there was nothing at all Kamijou Touma could do. He could
only say one thing to the identical pointy-haired high school boy.
“What are you!?”
“Imagine Breaker is just one card in your deck? You’re the one in control?”
There was obvious mockery in his voice.
A cracking sound shook the air.
A shocking pink light flashed at the corner of his mouth and an emerald one at the corner
of his eye. They were pretty colors, but did it mean he was unsure how to process the parts
that were not quite the body’s surface but not quite inside either?
And he used that extremely lifelike right hand to calmly pick up the phone that had fallen
on the thick ice.
“Don’t make me laugh, you snot-nosed brat. Once this right hand’s taken from you, what
have you got left? Who would ever accept you as Kamijou Touma now?”
There was a sound like long, wet hair splatting against a concrete wall.
Kamijou Touma’s right shoulder should have had nothing attached, but sky-blue threads
twisted around to fill that empty space. They formed what looked like powerful muscles
and quickly grew to the silhouette of a human arm. Once it was complete, he felt a
lessening in the intense pain causing sparks in his mind. The other boy had grabbed the
phone, but Kamijou used this new arm to reach for the wallet at his feet.
(An arm?)
But something scared him more than the bizarre visual.
It frightened him that his body was accepting this situation before his mind just because
the pain was gone. The ordinary decision to only believe what you saw with your own two
eyes could bare its fangs when you saw something truly unbelievable and hard to accept.
The thing he saw here was a part of his body.
It was not something to be rejected.
(This psychedelic thing is…my arm???)
He did not have time to question it.
With another bursting sound, the sky-blue arm opened back up like a giant maw. And it
worked to swallow up Kamijou Touma himself instead of the enemy before his eyes.
He no longer had anything to stop it.
Imagine Breaker had gone to the other boy.
“How can you call yourself Kamijou Touma without this?”
That other boy grinned while clenching and reopening those fingers.
As if to show off the power that should have belonged to Kamijou Touma.
“It’s time you learned firsthand what happens once it’s gone.”

- New Testament Volume 22 Reverse Chapter 2 Part 7