Who might be responsible for Harry disappearing on his 16th birthday in the fanfiction?

It could be Orochimaru. He was always interested in powerful jutsu and having Naruto, who is the Jinchuriki of the Nine - Tails, under his control would be a great asset. He might have planned a secret mission to snatch Naruto at birth for his experiments.

It could be Malfoy. He often has a vendetta against Harry and might have found a way to make him collapse during potions class, either by tampering with his ingredients or distracting him in a way that leads to a mistake in the potion - making process.

Maybe it's a result of a dangerous magical experiment gone wrong. Harry might be trying to test a new spell or a new form of magic that he read about in some old, mysterious book. But he doesn't fully understand it, and it backfires on him, leaving him badly injured.

There's also a possibility that it was an accident. Maybe Harry was exploring some ancient and dangerous magic, and something went wrong. A spell backfired and instead of achieving what he intended, it put a strain on his heart and led to the heart attack.

It could be a rogue Death Eater who managed to catch Harry off - guard. Maybe they were lying in wait and launched a surprise attack when Harry was alone, away from the safety of Hogwarts.

Maybe he discovers a dark secret about his past and decides to run away to protect his friends and Hogwarts. He could have found out that Voldemort has some new evil plan related to him specifically on his 16th birthday.

It could be Danzo. He was always power - hungry and had his own ideas about what was best for the village. He might see Naruto as a threat and find a way to take away his birthright.

It could be Dumbledore. If he is too focused on his grand plans against Voldemort, he might overlook Harry's well - being and needs, which could lead to Harry's death from neglect.

It could be a case of betrayal from within. Maybe someone who was thought to be an ally, like a wizard who was secretly working for the dark side all along. They could have used a powerful and unknown curse to kill Harry in the middle of the night without anyone suspecting them at first.

It could be Sasuke. Maybe Sasuke is in a bad mood on Naruto's birthday and rejects Naruto's invitation to hang out or something. Since they have a complex relationship, Sasuke's rejection would really hit Naruto hard.