It could be Draco Malfoy. Malfoy is always taunting Harry and his friends. His constant insults and attempts to undermine Harry might finally push Harry over the edge and make him lose his temper.
One possible reason could be that he's constantly being underestimated or not given the credit he deserves. In some fanfictions, his friends might take his contributions for granted, which could lead to his temper flaring up.
Arthur could be the cause. He often bosses Merlin around and doesn't fully understand Merlin's capabilities. His constant demands and lack of respect for Merlin's intelligence might push Merlin to the edge. For instance, when Arthur makes Merlin do dangerous and pointless tasks without any regard for his safety.
His own self - doubt could be the cause. Percy might be constantly thinking that he's not good enough to protect his friends or complete his quests. And one small trigger, like a friend making a harmless joke about his capabilities, could make him lose his temper because it hits too close to home.
Once there was a boy named Tom. He had a short temper. One day at school, a classmate accidentally bumped into him in the hallway. Tom immediately lost his temper, shouting at the poor boy. His face turned red and his fists clenched. But later, he realized his mistake and apologized.
One possible scenario could be when Merlin is constantly underestimated in Camelot. People might make fun of his clumsy appearance or dismiss his ideas. After a series of such incidents, he loses his temper. For example, if Arthur repeatedly ignores Merlin's warnings about a coming danger and then blames Merlin when things go wrong, Merlin might snap.
He might accidentally reveal his identity. In some fanfictions, when Peter Parker loses his temper, he can't control his actions or words as well. This could lead to him slipping up and showing his superpowers in front of the wrong people, which would put his secret identity at risk.
In this fanfic, it might be that Percy is in a high - pressure situation like a really difficult quest. Things keep going wrong, his team isn't listening to his plans, and he just loses his temper out of frustration. He starts yelling at his teammates, accusing them of not taking things seriously enough.
It could be Voldemort. Voldemort might use Harry's love as a weapon against him. He could either kill her or turn her against Harry. Another possibility is Draco Malfoy. Maybe Draco tricks Harry's love into thinking he's a better choice. Or it could be a new, unknown dark wizard who targets Harry through his love.