Perhaps an enemy from Rito's past. This could be someone he wronged unknowingly or a foe from a previous battle who has been waiting for the right moment to strike back, and stabbing seems like a brutal way to get revenge.
There's also a possibility that it's a character who is being controlled by a greater evil force. The real antagonist might be using this character as a pawn to stab Rito in order to further their own dark plans, like taking over a kingdom or destroying a group that Rito is part of.
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.
I'm not sure specifically as there could be many different interpretations in such a fanfiction. It might involve a character named Rito facing a stabbing incident, perhaps in a battle, a crime scene, or a situation of betrayal.
It could be a rogue angel. There are always some angels in the Supernatural universe who oppose Castiel's ideas or actions, and they might see stabbing him as a way to get rid of him or send a message.
It might be a jealous rival. If Sakura is popular or has something that others envy, like a special talent or a relationship, a rival could have stabbed her out of envy.
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.
In 'naruto gets stabbed fanfiction', it might be a new enemy that has emerged, someone with a powerful and unknown jutsu. This enemy could see Naruto as a threat to their plans and decide to stab him to eliminate him. Or it could be a character who has been corrupted by dark power, like a former friend or ally who has fallen to the dark side and no longer recognizes Naruto as a friend.
It could be an old enemy of Nicole's from her police work. Maybe someone she put in jail before who has a vendetta against her. This person might have been released recently and is out for revenge.
Well, in 'naruto gets stabbed fanfiction', it could be that Naruto is exploring a new and dangerous area where he encounters a powerful and hostile ninja. In the heat of the fight, this ninja manages to land a stabbing blow. It might also be that Naruto is weakened due to some prior injury or chakra depletion, making him more vulnerable to being stabbed by a regular enemy. And, there could be a situation where Naruto is under the influence of some sort of genjutsu that impairs his ability to defend himself and thus gets stabbed.
It could be a jealous rival duelist. Someone who has always been in Kaiba's shadow and saw stabbing as a way to eliminate the competition.
It could be a jealous gamer. Someone who has been envious of the Game Shakers' success and sees Hudson as an easy target to get back at them.