Crup: The Crup resembles a Jack Russell terrier, which is Ron Weasley's patronus, so write a fic about Ron.
The Order knew that someone was a spy for the other side. Someone was feeding Voldemort information about who would strike where, when Harry would be various places, when Voldemort could strike for the maximum damage. Suspicion fell at first on Snape, but when Snape showed up one wintry evening, pale as a corpse and coughing up blood, admitting that Voldemort had discovered his true allegiance and he'd barely gotten away in time, well, they had to find another culprit.
Still, no one could have guessed the truth that was revealed sometime in late May, when Ronald Bilius Weasley stood at the gates of Hogwarts, Death Eater mask in one hand, hair blowing about his face, and proclaimed that his Lord approached, and that they must all surrender to the Darkness. His freckles stood out like a brand, but his voice was steadier than Harry had ever heard it.
Hermione cried, whispering "why" to herself like a broken record player, but there were no answers. There never were. The Battle of Hogwarts was fierce and bloody. When it came down to it, Harry and Voldemort dueling each other, Hermione found Ron, his leg pinned by falling bits of rock, blood painted on the side of his face.
"Why?" she asked him, wand trained on him.
"No one suspects a Weasley," he said, his mouth twisting, and slipped into unconsciousness. It wasn't until months later, after he'd been trapped in St. Mungo's, that they discovered he'd been under the Imperius Curse all year, cast by Bellatrix Lestrange when he'd visited Diagon Alley alone.
"I'm sorry," Hermione said when he was discharged, but he had no words for her. Not yet.
He did have a smile, and he bestowed it upon her with heart-stopping sweetness before limping off, his hand firmly grasping his blackwood cane.
