For many days, Clary seemed to have successfully faked out a nice relationship with her brother; afterall the only way to get closer to him and his plans was to put her trust in him and show him what a dependable and supportive girl she was.
However things all got screwed up when Clary discovered her precious lost faerie ring in Sebastian's room - and also got caught redhanded, trying to steal back the ornament.
Sebastian admitted to his sister about how the Seelie Queen had spied over the conversations that had taken place through the rings (much to Clary's horror, who then remembered Leah's warning in the library), and then he suddenly took the topic to a very, very incesteous edge when he attempted in kissing Clary.
Clary, now no more the scared little girl she had been when she had first faced Sebastian, had the senses to kick, hit, claw and slap - just to somehow escape her brother's wrath.
But Sebastian was quicker, stronger and more resistant of injuries. He calmly smiled over the few punches Clary had thrown at him, made a sleek comment about how she had improved on her battle moves, and then threw her over to the bed.
Clary attempted in hitting him once more, but Sebastian quickly grabbed her hands, pinned them behind her back and threw Clary's body on the bed, making the girl yelp. Flashing his much hated evil smile, Sebastian dramatically climbed over Clary's struggling body, gently draping a leg across her hips.
Clary tried shouting or spitting at him, but Sebastian sealed her lips with his own mouth, lingering there for a moment before slowly taking the kisses down her neck.
Quiet, but sharp and lethal, a voice pierced through the atmosphere around them. A simple"Jonathan Morgenstern."and that was all it took for Sebastian to put an abrupt end to his naughty romance.
He remain frozen in thought on top of Clary, as underneath him, his sister seeked points for attack. "Hey, Leah." Sebastian said, slowly getting up from his initial position, meeting the intruder's face almost awkwardly - as if he were embarrassed to do so.
Clary breathed heavily and stared at Leah - who was looking at Sebastian with hard cold grey eyes.
"You violated the deals." Leah said, sounding oddly robotic in nature, and intimidating regardless.
"Come on!" Sebastian cried, like a child who had just been told that he wouldn't get his pocket money this month. "I was just having fun. I'm sure you don't mind who Clary sleeps with; because indeed, she's got a very long and extremely pornographic -"
"Perhaps I don't mind her intentions, but I definitely mind yours." Leah cut him off sharply, much to Clary's relief, before Sebastian could start bragging about her love life. "We had a pact that clearly stated that you were not supposed to harm Clary - physically, emotionally or sexually. Yes, it did state all three of them." Leah said the last sentence with particular emphasis.
Sebastian only chuckled. "That was hardly sexual. And besides, what can you do now? I shall continue with my plans, and very soon when I have shown you the antidote I have created, I shall also have your full support. Then....then, you know we're gonna be unstoppable."
Leah slowly said "You think you are best shadowhunter that was ever created; you think that you are indestructible. You overestimate yourself Jonathan; you have been destroyed before - and you will be destroyed again. You have violated our deals, and so have I."
A horrible sound of exploding bricks and concrete and wood was heard then, and Sebastian startledly looked beside him to see a massive - seemingly irreparable - hole in the wall of the room.
Clary had successfully used Sebastian's distraction to carve an unfinished opening rune on the wall - so that the wall went exploding, destroying the dimension and its feature of being able to move around the world.
On realising the impact of the injury, Sebastian let out an angry cry; Clary swiftly jumped out of the hole she had created, but her brother was far too quick. He grabbed her by the arm and roughly dragged a screaming Clary back into the room.
Sebastian began to aggressively swear and hit her in the jaw once; Clary hoped Leah would help - considering how serious she had been moments ago about the fact that her brother had 'violated the deals' they had made for themselves.
But Leah simply said "You are only wasting your time, Jonathan. My troops will arrive in no time - and I've got a pretty diverse army, mind you." and saying so, she gracefully went out of the hole in the wall.
Realising that Leah was about to go and shoo away the Shadowhunters he had gathered the support of, Sebastian ceased tackling Clary, and barked at her to get into her robes and then make a portal to the Seventh Sacred Site.
Scared, Clary did what she was told to, not deliberately playing things slow since she knew that an angry Sebastian was not a good sign. A portal was created, and Sebastian practically dragged himself and his sister into it.
Immediately after Clary and Sebastian had reached their destination, Sebastian firstly made sure that everything was going the way he had planned them: those who had decided to offer their support to him were on their places, there were no suspicious activities around, and Jace was giving his speech on the stone platform.
If Leah had decided to go till the ceremonial site on foot, she would indeed take a very long time to reach there from where she started. Sebastian mentally chuckled at the thought and at his intelligence of using a Clary-made Portal to travel. He quickly set up the apparatus, summoned Lilith and then created the Infernal Cup - much to his audience's awe.
A haggard and tied-up Amatis was called, forced to drink from the Cup and Turned into a dark warrior, for the people to feast their eyes on. Clary gasped and gulped in the background as each of the good Shadowhunters came and got Turned into evil.
Clary had decided that she couldn't watch any more of this nonsense and was planning to escape the Site, when an Endarkened man cried out. Somehow, unseen to everyone, he had been stabbed through the stomach by a seraph blade.