One of his friends could be the cause, accidentally. Maybe a friend makes a deal with a shady character without realizing the consequences. This deal ends up putting everyone in danger, and Yami gets mad at his friend's naivete and the mess they've created.
In a 'yami gets mad fanfiction', it could be that Yami discovers a betrayal. Suppose he trusted someone deeply in a magical world they inhabit together, and that person has been secretly working against him. This realization makes him extremely mad, and he has to decide how to deal with this traitor while also protecting his friends.
A new player on an opposing team could be the cause. This new player might be overly cocky and disrespectful towards Kuroko during a game. They could constantly taunt him about his lack of presence, which finally gets on Kuroko's nerves and makes him mad.
It could be Vegeta. Maybe Vegeta does something really underhanded like stealing Goku's senzu beans or trying to take all the credit for a victory that Goku also contributed to. Goku, being a fair - minded person, would get angry at such behavior.
Another possibility is a jealous rival. Maybe there's someone who is jealous of Gohan's power or his relationship with certain characters. This rival could do things to annoy Gohan and push him to the edge of his temper.
It could be Perry the Platypus, in a really unexpected turn. Maybe Perry has to keep some secrets from Phineas for his spy work, and one day those secrets are revealed in a way that makes Phin feel betrayed. This would definitely make him mad.
In the fanfiction, Yami suddenly falls ill. His friends notice his pale face and lack of energy. They quickly bring him to a healer. The healer diagnoses him with a strange fever. As Yami lies in bed, weak and delirious, his friends take turns to look after him, bringing him warm soup and medicine, hoping for his speedy recovery.
It could be that Ron's actions were causing problems within the Order of the Phoenix. If Ron was being stubborn and not following the plans or orders set by the Order, which Sirius is a part of, Sirius might get angry. He would be frustrated with Ron for not being a team player and potentially endangering the mission and the lives of others.
In the fanfiction, Yami could be cursed by an enemy, which makes him sick. His strength starts to fade away. His friends notice the change in him and start to investigate. They discover the curse and search for a way to break it. Along the journey, they meet various characters who either help or hinder them. Yami's condition worsens, but his friends don't give up. Eventually, they find a powerful mage who can break the curse, and Yami is cured, learning valuable lessons about the power of friendship and the importance of being vigilant against enemies.
In some yami and seto fanfictions, Yami might be depicted as the more mysterious and powerful one. Seto, on the other hand, is often shown as the proud and wealthy character. Their interactions can range from intense rivalries to unexpected alliances or even romantic undertones. However, in lemon fanfictions, these characteristics can sometimes be exaggerated for the sake of the plot.
Physical hurt might come from a powerful magic spell. Let's say an evil sorcerer casts a spell that drains Yami's energy and causes him pain. His struggle to overcome this and regain his strength could be a central part of the fanfiction.