Another character who has a personal vendetta against Clary or her family. For example, a jealous rival or someone who blames her family for past misfortunes. This would add a layer of drama and personal conflict to the fanfiction.
Alec might also come to her rescue. As a skilled Shadowhunter, he has a strong sense of duty. Plus, he's part of Clary's group, and he wouldn't stand by and let a fellow Shadowhunter be attacked by a demon.
Perhaps an older, disgruntled member of Fairy Tail who has some hidden grudge against Natsu. Maybe Natsu unknowingly got in the way of this member's plans or achievements in the past, and now they see an opportunity to get revenge. They might use their knowledge of Natsu's fighting style and weaknesses to launch a surprise attack within the guild.
Well, she might be rescued by her friends. In many fanfictions, her Shadowhunter comrades would come to her aid, using their unique skills to fend off the attackers.
It could be a criminal he put away in the past. This person might have escaped from prison and is seeking revenge on Deeks for ruining their life.
A third plot twist could be that the attack is a diversion. While Clary is fighting the demon, something much more significant is happening elsewhere. Maybe a powerful artifact is being stolen or a key location is being infiltrated, and by the time she realizes it, she has to race against time to stop the real danger.
It could be a jealous rival. Maybe someone who is envious of Rito's status or abilities and decides to take him out by stabbing.
Another possibility is a character who has been possessed by an evil spirit. They may not be in their right mind when they stab Merlin. This could add an element of mystery as Merlin has to figure out not only who stabbed him but also how to exorcise the spirit from the person's body.
Well, often it's Clary, Jace, and Simon. Simon is Clary's best friend, and in some fanfictions, his relationship with Clary can make Jace a bit jealous, but at the same time, if Simon seems too friendly with another girl, Clary might also get jealous in a different way. It's an interesting triangle - like situation that can add a lot of drama to the fanfiction. And of course, Magnus Bane might also make an appearance and his flamboyant personality could somehow be involved in situations that lead to Clary's jealousy.
Another option is a group of demons who have a vendetta against Clary or the shadowhunters in general. They might use torture as a way to break her spirit and gain information about the shadowhunters' defenses.
Maybe it's a new Gryffindor student who has been influenced by the wrong people. This new student doesn't really know Harry well and is easily convinced by some anti - Harry propaganda within the school. So they become the main antagonist in the story and lead the attack on Harry.