Disclaimer: I obviously own nothing from the Harry Potter series. Possessed is fanfiction and written purely for fun and no profit.
Harry's been having nightmares.
After having shared a dorm room with Harry for quite some time, Ron knows that it isn't normal for Harry to have nightmares.
At first, Ron notices Harry tossing and turning in his bed throughout the night, but by the time morning comes around, Harry is unable to recall what he'd dreamt about the night before.
As weeks go by, Harry's nightmares worsen to the point that Harry is often unable to sleep through the night. Harry still can't recall what his nightmares are about once he wakes, but feelings of dread and anguish linger. On those nights where Harry does manage to sleep through the night, Ron can hear him muttering. Occasionally, Ron can pick up what Harry's saying but not often.
Once Harry's nightmares lead to him pleading in obvious distress to someone to not harm himself or his loved ones, Ron starts to put up a silencing spell around his bed. Ron finds that he can't sleep well himself with the knowledge that Harry's suffering in the bed next to his. Whether Ron wakes Harry from one of his nightmares or waits until Harry wakes up naturally, Harry can't recall his nightmares, just the feelings that they leave behind.
Hermione's been acting strange, too.
For no apparent reason, Hermione's become distant lately, having no interest in spending time with Harry or Ron. There hadn't been any sort of falling out, which makes the situation baffling. Ron tries to get her to help him figure out what's wrong with Harry, but she just gives him a dead stare whenever he bothers to ask her anything.
Hermione also isn't as prone to answer the professor's questions in classes unless directly asked for the answer. Her normal enthusiasm is gone.
Still, Hermione spends all her free time in the library, more so than before now that she's no longer spending time with Ron and Harry.
Ron's worried for both Harry and Hermione.
Ron can tell that both Harry and Hermione are getting worse as time passes, and he doesn't know what's wrong with either of them, but he knows that something is most definitely wrong.
Ron's able to convince Harry to go to the Hospital Wing, but the mediwitch determines that nothing is wrong after running various diagnostic tests on him. This just makes Ron worry even more.
Hermione isn't even willing to speak to Ron, let alone follow his advice or requests.
Eventually, Harry's able to recall his nightmares and speaks to Ron about them. He sees himself and his loved ones getting captured, tortured, and murdered by some dark wizard calling himself Lord Voldemort. Neither Ron nor Harry have ever heard of someone named Lord Voldemort. Ron encourages Harry to send a letter to his parents to see if they know of this name, but the letter they send back makes it clear that they also have not heard of this man. Research in the library proves fruitless as well.
In the middle of the night one night Ron wakes to see Hermione standing over Harry's bed, looking at him intently and with a malicious expression on her face. Before Ron even has a chance to grab his wand, Hermione sends a bright green killing curse at Harry. Unthinkingly, Ron immediately stuns Hermione and summons her wand to him. He screams when he goes over to Harry and he's unable to feel his pulse.
Ron's still-living dormmates wake up at that. Neville runs off to grab a professor while Dean and Seamus stay behind as backup to make sure Hermione doesn't go anywhere.
Sometime later, Headmaster Dumbledore, Professor McGonagall, Professor Snape, and Madam Pomfrey arrive. Hermione and Harry are removed from the dorm, and the boys are questioned. Ron tells what he knows, which isn't much, but he sees a slight glint of recognition in the headmaster's eyes when he mentions the name Lord Voldemort.
Professor McGonagall goes to the girls' dorm and presumably questions Hermione's dormmates and goes through Hermione's belongings. When she leaves the girls' dorm, Ron notices that she has a black leather journal in hand that she hadn't had when she'd entered.
Within a single night, Ron's lost his two best friends.
Ron attends Harry's funeral and sees Harry's parents and godfather sobbing over Harry's casket. He has a hard time not falling apart too.
Headmaster Dumbledore informs Ron that Hermione had been possessed by a dark artifact and has been placed in a highly secured room at St. Mungo's to see if the possession can be undone. If it can't, she'll likely serve a life sentence in Azkaban, or rather whatever is possessing her will. If it can, then she'll likely need years of treatment to overcome the possession. Either way, it doesn't look as though Hermione's going to be free any time soon.
Ron never forgets about Harry and Hermione, but the pain lessens as the years go by. He heals and makes new friends and evolves in ways he'd never thought he would. He makes his family proud.
Shortly after Ron passes auror training and becomes an auror, a name starts to pop up, again and again, that Ron only ever heard of from Harry all those years ago: Lord Voldemort.
He knows now that he was right to worry about Harry and Hermione all those years ago and that Hermione's possession most certainly was not undone.