Perhaps Voldemort himself. He might see Draco as a tool, and Imperiusing him would allow Voldemort to have more control over the Slytherin side of Hogwarts or use Draco to spy on Harry. Another possibility is a rogue wizard who has a grudge against the Malfoy family and wants to cause them trouble by making Draco do things he wouldn't normally do while under the Imperius Curse.
Harry might be shocked at first. He'd probably feel a sense of urgency to break the Imperius Curse. He could also feel a bit of pity for Draco, as Draco is being controlled against his will.
One idea could be that Draco is put under the Imperius Curse by Lucius to perform a dangerous task at Hogwarts, but Harry somehow senses something is wrong and tries to save Draco. Another could be a backstory where Lucius used the Imperius Curse on others during his time at Hogwarts, and Draco discovers this and has to deal with the moral implications. Maybe also a plot where Harry uses the knowledge of the Imperius Curse on Lucius to gain information about Voldemort's plans, and Draco gets caught in the middle of this power struggle.
Actually, Harry could be the most interesting character to develop in this fanfiction. His relationship with Draco is already full of tension. When he discovers Lucius using the Imperius Curse on Draco, he has to decide whether to help Draco or use the situation to his advantage against the Death Eaters. His moral compass and his natural inclination to do the right thing would be put to the test.
The plot could involve Harry suspecting the Death Eaters or some other dark forces of being behind the Imperius Curse on Draco. So, he starts to investigate secretly, which might lead him into dangerous situations. Maybe he discovers a connection between Draco's Imperius state and a larger, more sinister plot against Hogwarts or the wizarding world in general.
In fanfiction, the Imperius Curse can be used to create really exciting plotlines. For instance, a character could be put under the curse and made to do something that goes against their nature. This could lead to a moral dilemma for the other characters. Do they blame the person who was cursed or the one who cast the curse? It's a great way to explore themes of responsibility and free will.
One great fanfiction could be 'The Malfoy Redemption'. In it, Draco is under the Imperius curse cast by his father Lucius, but Harry Potter steps in to break the curse. It shows a complex relationship between the characters.
One interesting Imperius Curse fanfiction could be about a character who manages to break free from the curse in a unique way. For example, they might use their love for another character as a sort of shield against the curse's control. In many fanfictions, the Imperius Curse is explored in different scenarios than in the original books. It could be set in a time after the main events of the series, where a new dark wizard tries to use the curse on the heroes again.
Well, in many such fanfictions, Harry might be shocked at first. He probably feels a sense of pity for Draco too. After all, being under the Imperius Curse means Draco has no control over his actions.
In the fanfiction, if Harry discovers Draco is imperioused, he might first be shocked. Then he would probably try to find a way to break the imperius curse. He could seek help from the teachers at Hogwarts, like Professor Snape or Professor Flitwick, who are knowledgeable about dark magic and counter - curses.
Maybe in some fanfictions, Draco is driven by a sense of self - preservation. He might believe that Harry will eventually turn against him or the Slytherin house. He could think that by killing Harry, he can secure his own place at Hogwarts and in the wizarding community. Additionally, it could be part of a larger plot by Voldemort or his followers. If Draco is forced to join Voldemort's side, he may be given the task of killing Harry to prove his loyalty.