In book four City of Fallen Angels Jace fears he will harm Clary due to some scary dreams he is having. We later learn these dreams are being planted in his mind from a female demon who is trying to bring Sebastian back from the dead (and is trying to make more demon/human babies but failing miserably). In order to accomplish this the demon needs the help of Simon and that is why he was being attacked all the time. Simon is now living with Kyle (Jordan) who happens to be a werewolf and Maia's ex-boyfriend. Maia and Isabelle both break of their relationships with Simon, however Isabella starts coming back around and Maia reconnects with Jordan. Meanwhile the demon Lilith gets a hold of Jace, puts him under her spell and has him kidnap Clary. Jace, Clary and Simon end up on a rooftop with Lilith who is trying to bind Jace and Sebastian together and bring Sebastian back to life. However the trio manages to slay Lilith and the rest of the gang and the Clave come to the rescue and all seems well again. But while everyone is distracted Sebastian manages to take hold of Jace mentally, now Jace is under his control and they disappear together.
Since the sixth book in this series, City of Heavenly Fire, is due out until 2014! I am going to detail City of Lost Souls instead of just summarizing it. Okay Jace and Sebastian have disappeared and are nowhere to be found. However they come for Clary in the middle of the night. They want her to leave home and join them but a fight ensues with Luke and the pair run off.
Clary knows that they will be back though and she has on her a pair of faerie rings she promised to collect for the faerie queen. The rings allow the wearers to communicate with each other telepathically and Clary hatches a plan to go with Jace and use the rings to contact Simon to let him know where they are. The plan works for a short while until Clary takes some faerie drugs and wakes up without her ring. Jace has become a mindless lapdog for Sebastian and Sebastian keeps getting creepier and creepier. Eventually Clary learns of Sebastian's plans and is able to contact Simon with the information.
Meanwhile Simon decides to call on an angel to give him a weapon he can use to separate the bond between Jace and Sebastian. The angel agrees in exchange for removing the Mark of Cain from Simon. When they are done Simon collapses and Isabelle rushes to him thinking he has died. Isabelle is starting to fall for him.
In the end Sebastian has raised the demon Lilith and is using her blood, in a second mortal cup he had forged, in order to change some of Valentine's followers into demons. Our band of heroes and Luke's wolf pack make it just in time to save Clary from being forced to drink from the cup but it is too late for Luke's sister who was kidnapped from her bed. A battle ensues, Simon cannot make it to Sebastian through the crowd so Clary grabs the angel's sword from him and stabs Jace through the chest. She took a huge risk but it works out in their favor and any evil inside of Jace is killed off. However in the chaos Sebastian disappears yet again.
Quotes:
“That thing that looks like me but isn't? He'll burn down the world if Sebastian wants him to, and laugh while he's doing it. That's what you're saving, Clary. That. Don't you understand? I'd rather be dead—”
and
“I stabbed you. With a massive sword. You caught on fire."
His lips twitched, almost imperceptibly. "Okay, so maybe our problems aren't like other couples.”
