It’s theory time!
The Arbiter was supposedly shut down a while ago, according to what denizens at Oribos say. My guess is that the thing that flew down (the red looking soul) and struck the Arbiter was either: A.) Argus’s soul or B.) Azeroth’s World Soul when Sargeras struck the planet with his sword. It sounds silly, but it would give it a good tie-in with Azeroth and earlier plotlines from Legion and BfA!
What do you guys think? 
The real big bad is deep inside her, she will be the raid boss at the end to rid her of the evil within her!
I dunno really, either way I am really enjoying the story so far.
Azeroth is alive still so… that’s that theory debunked.
Argus’ soul makes more sense. The Dreadlords could’ve manipulated his soul to go to the Shadowlands instead of remaining in the material plane the way other Titan souls had. Argus experienced untold ages worth of horrid torture as well. The Arbiter judges mortal souls, and she experiences their entire lives in an instant. Experiencing all the pain, anguish, and agony of Argus in an instant seems like it’d have been enough to BSOD her.
So the dreadlords basically blue screen of death’d her? I kinda like that idea
Oh derp that’s what BSOD meant. I’m dumb
That seems the most plausible to me. Denathrius’ Dreadlords playing the long con and setting up all kinds of contingencies. The defeat of the Legion and death of Argus being one such contingency makes sense.
We actually know what happened though. I’ll hide it for those you haven’t finished the main campaigns yet, but it’s not Argus.
It was Sire Denathrius. The red energy blast in the cinematic was his direct attack on the Arbiter to deactivate her and fuel his new Jailer friend. He openly takes credit for it at the end of Revendreth, everyone knows now he was to blame for all of this.
Ah, thanks for the clarification! Well, that’s much simpler, which is probably better.