
Grolwgra ran off, so his fate was up in the air right then...
As for if what you just wrote is possible... well, it both isn't and is. Join me for my counter (possibly) surprise....
Before light extinquishes her vision, she sees the furred body of Grolgra disappear into the darkness, although twin streaks of fiery red rush after him. Then the surprised look of the miniature Chaos as he turns to look at her.
And then she loses all connection with the world, as the light becomes so bright. The last thing she sees is the small point of darkness that is the real Chaos. The last thing she sees before the light becomes painfully bright is his look of anger, the same one he always has. And then, at the last moment, she hears his laughter.
She had done this before...
Blackness.
"No, you promised!" says a petulant voice.
Light
Slowly light comes to the darkness, a little pinprick that finally forms into the glowing white hot pillar of corbum again. Petal doens't need to shade her eyes, because the brightness does not dazzle her, nor infact illuminate anything of the nothingness of her surroundings. It was hard to see the runes on the pillar anymore, but thankfully the distracting blaring noise was now gone. The four runes were still attached to the pillar, singing a slightly discordant but not unpleasant tune now.
"You promised! You said we'd take everything back, then you went and made it all go away!" The high pitched petulant voice was behind Petal. Turning around, mini-Chao's cloak was barely smouldering, and he had a crushed look on his soot stained face. Brimstone tears were marking his cheeks. "I can't make anything of this!" he wailed. His helmet was long since gone, but Petal could imagine it wobbling on his head anyway.
And then the darkness behind the miniature Chaos took form, and the real Chaos strode forwards, picking up the small version of himself with ease. One taloned glove wrapped around the miniature's shoulders, the other around the minaiature's mouth.
"You always do this," said Lord Chaos, crowing with glee. "You always do the thing you could never do in real life. Destroy the pillar's supports so others could succeed in destroying Chaos. Let Grolwgra go free. Haha! You are trapped here forever, a perfect prisoner in your own mind trying to undo your mistakes. Such a joy to watch, each and every time."
Lord Chaos shook the struggling prize in his hand, much to the muffled anger of mini-Chaos. "You know what this is? What this thing truly is? It is your memory of Lord Chaos. And how you have tried to humanise him, make him weaker and smaller in your mind. Harmless. You don't want him to be as terrifying as he truly is. You don't want to think of the terror you allowed to destroy the land. You don't want to know that, even now, he looks in upon you as you hang on his wall, trapped in a mirror while the world burns and freezes upon his mighty whim."
Lord Chaos - or whatever it is - laughs maniacally while the mini-Chaos struggles ineffectually. "And as you may guess, that makes me Lord Chaos's power - the magics binding you here forever. I don't think you have ever bridged the gap between one dream of your death and the next before." Lord Chaos's power shakes the small figure again, and the small figure cries and screams as much as the straining arms allow. "Perhaps you have truly started to accept your prison for what it is - a fitting punishment. Perhaps you are starting to accept Chaos, for you know he is alive and well. Perhaps this is the beginning of you seeking his favour." Again the laugh. "For now you have left yourself this precious moment of awareness which which to make a real, informed choice. A choice you know you will fail at. How masochistic."
Lord Chaos nods to the pillar. "There is the pillar. Go on, destroy the runes that bind it." Then Lord Chaos's power shakes the small Chaos for what could be the last time. "However, this time, your 'friend' does not escape. You know I will snap his neck before you can act, either way, and end this little interlude. So what will it be? Are you the hero you always choose to be, saving everyone? Or will you prove, even now, that you cannot do what must be done. Even for an illusionary creature, even for an aspect of Lord Chaos himself - do you have the courage to turn your back on it? Or will you prove, just as you did in life, that you have not the stomach nor courage to be great. Will you once more die needlessly and for nothing?"
Petal struggles to take it all in, but not because of the concepts Lord Chaos is spouting. It is because she recalls odd little things from the dungeon. The little burned notes all around. Fragments of messages about the pillar, and about the runes. Except now, she realises it was a trick of the mind to think they were from Lord Chaos. For they were in her own handwriting.
She was leaving herself a message.
Even as she was locked in the memories of her death, trapped in some prison of death for Lord Chaos's amusement, she had apparently left herself a message. Who knopws how many times she had to relive these memories for it to take hold, for her to shape her prison. Still not enough for the memories not to be fragmented, 'burned'. Still, she had left herself a clue. A clue to a way out? Yes, she felt it - the pillar was a message to her from herlsef. Or at least the runes were. 'Lord Chaos', or rather his power keeping her here, had tried to disguise it. Even now, it was seeking to force her to destroy the message instead of read it properly.
She had won! All Lord Chaos's power could now do was force her to snatch defeat from the jaws of victory by giving her a false choice, a false choice to destroy the message or waste time saving a figment.
And yet... she had only won this opening by empathising with something so foul as the memory of Chaos apparently. Petal could feel it, this place was only open because of her final thoughts on the diminuative High Lord. The power of Lord Chaos could easily snap mini-Chaos's neck at any time and end this chance. She had to move fast. Was it wrong, given the circumstances, to turn her back on something helpless? Was there really any reason to attempt to save the miniature Chaos?
Petal couldn't recall her life outside of this strange nightmare. Had she been forced to do this before? Sacrifice someone like Grolwgra for the greater good? Was Lord Chaos's power trying to trick her through guilt to not see the message? Or was this who she was. Would she be betraying herself by allowing someone to die - even loathsome and imaginary. Would she lose a part of herself in this mirror finding a way out? Was that too great a price to pay?
Lord Chaos's power apparently laughed at her indecision. However, it obviously thought it had won regardless. She knew the power was not aware of her knowledge of the pillar's real meaning. However, did that matter?
What should she do?
And there we have it! Wow, didn't think we'd hit the end so soon! However, your post was brilliant, so I just hope where this has led to is a decent pay- off for you. I did warn you I might do a 'trapped in the mirror memories' thing, and it was the easiest way to a) retcon any mechanical stuff, b) explore and shape petal's backstory while being able to have you still choose most of it at the end, and c) give you a real chance to have Petal save herself.
Anyway, for a discussion on what it all was going to / does mean for Petal, her backstory, etc, join me from tomorrow onwards in talking it all through and figuring out where she goes on from there.
But that's after your next post. Next post, all you need is a simple choice really, no clever plans, etc - which I hope is a cool character choice for Petal that you can then explore. Is petal the sort of person who will do what needs to be done - or has she at least been forced to become that person? Or is she the sort of person who - even if they are imaginary and evil friends - can't leave a friend to their fate without maing a gesture of sacrifice?
No right answer here, I'm excited to see which you go with, and your/Petal's thinking in making it 
Good luck - see you on the flip side for some fun discussions (I hope).