Watching the world burn was never something Clary wanted to see. I would say it was something nobody wanted to see but, with people like Sebastian, that would be a false statement.
The dream Magnus had had a few weeks ago had come true. Clary fell asleep in Alicante and woke up in a hard bed, blankets tucked under the mattress almost entrapping her in their folds.
As she unwound the sheets from her feet, she decided that something felt off. Something dark and ominous lurked in the distance known as the future. She slammed open the door and ran down the long hallway it opened in to.
After running for hundreds of feet, she came into a huge living area surrounded by glass windows. She looked down and what she saw made her scream. The city, her home New York, lay in burning rubble while the streets literally ran red with the blood of those fallen.
As she looked closer, Clary saw that there was gold in the red: ichor. She ran again. Up a flight of stairs until she burst through into the cold, bitter air of the artificial permanent night.
As the stench of rotting flesh and hundreds of gallons of blood hit her nose, she vomited. She continued until all she could do was wretch. She scrambled to the edge of the roof and stared at the destruction.
In the distance, she saw a group of people. At first she was ecstatic: There are survivors! Then she realized they were being slaughtered. Shadowhunter after Shadowhunter fell to the ground, spouting blood, cut limbs hanging from shreds of flesh and muscle.
She heard a chuckle behind her and turned to see her brother standing there.
"You. YOU! LOOK AT WHAT YOU'VE DONE!"
Sebastian smiled, his teeth only a shade whiter than his skin. "Isn't it beautiful?"
"Beautiful? You're a monster! You killed millions of innocent people and their families! How could you—" She stopped, realizing that this was Sebastian. Of course he could do this and then laugh about it. She looked down and saw he was holding a sack.
Sebastian followed her gaze and his perverse smile widened. "Oh. This is for you to finally convince you that you belong to me. You see: before you belonged to Jace. Now: Jace is no more."
His words slapped her and she started shaking her head, tears rushing down her face as his smiled widened impossibly further. He advanced towards her and she crawled backwards until she met the very edge of the building.
He opened the sack a dropped something into her lap.
It was Jace Lightwood's head. The Eyes were gouged out, mouth cut at the corners to make him smile for all eternity, hair cut to the scalp, some of the scalp itself missing.
Clary threw the head to the ground and scrambled sideways until she stopped and curled up, rocking back and forth as the tears fell and her mouth opened in a silent scream.
"Come now, Clary. It's not that bad. You still have me." He grabbed her and let the shadows swallow them.
The black marble of the throne room glinted in the dim lighting of the fires. Inside each torch was multiple faces, preserved so as to not kill the soul but only torture it with fire. Fear and cuts etched into each face, they all screamed a silent, pleading scream.
No longer did anyone want to survive. Now they all just wanted to die and be spared of Sebastian's wrath.
"More wine, my dear?"
Sebastian looked at his "wife" as she sat on her throne in her robes. Her head cocked to the side, eyes wide and empty. Sebastian sighed. After Jace's death (everyone's, really) she went insane. He saved fragments of her sanity but she didn't desire to be saved.
Clary nodded slightly and a demon brought her Lilith's thick, lumpy blood mixed with wine. Clary drank, the corners of her mouth turned black from the demonic liquid.
Her eyes suddenly lightened, her dull green blazing. She hissed and jumped towards the demon. The demon screeched and attempted to flee. It never got the chance for Clary ripped its head off with her bare hands and then crushed it between her hands like a child would a butterfly.
Sebastian smirked.
At least she still responds to that.