Analysis:What Hurt Satoru during the Power Outage

The 2nd day in SPHIA, Satoru is attacked by an unknown shadow which stabs his left arm. Because of the power outage, this shadow wasn't recorded by the surveillance cameras, leaving their identity a mystery.

Sorting out the scenario
First of all, let's summarize what happened to Satoru's body before and after he was stabbed.

[Kokoro's Consciousness] Earthquake and power failure occur.

[α's Consciousness] Satoru's arm seems to have been stabbed at this time.

[Satoru's Consciousness] Satoru has severe pain in his left arm and encounters a figure with the knife.

[Kokoro's Consciousness] Kokoro flees desperately from the unseen assailant. Power returns.

Identity of the Shadow
Now we have a rough idea of who attacked Satoru.

It was the fetus ω which controls Inubushi's body.

Hypothetically, if the culprit was α which controls Satoru's body, we would have expected Satoru to be holding the knife and the shadow would have been stabbed in the "fight" rather than Satoru.

If Satoru chooses to fight the figure, he is killed and the figure kisses Satoru. The narrative mentions long hair and "the 'shadow' darker than the night".

This is undoubtedly Inubushi.

If you look closely at the hands holding the knife in the CG, they also show that it's Inubushi with high probability. It's pretty easy to confirm in the album, just look closely at the 4 knife CGs. Some are drawn but not actually used in the main story. CG #1 is unused (Yuni?), #2 is the assailant (Inubushi), #3 is unused (Utsumi), #4 is the assailant in the basement (Enomoto).

Inubushi at this time probably didn't think of killing Satoru. After the transfer, she found herself holding a knife in the dark. Satoru misunderstood and rushed at her, resulting in him getting stabbed while they were struggled.

Summary
Inubushi is the shadow at the time of the power failure. However, it's likely ω's consciousness stabbed Satoru's left arm.

But why was Inubushi holding a knife in the first place? Perhaps Hotori was preparing something in the kitchen, or ω found it when she took over.