A/N: As always, I don't own the Potterverse. In this particular case, I also don't own Hasbro, Wizards of the Coast, or the game Magic: the Gathering, though I do own a copy of the card Burning Wish.
She wished for a weapon, but not for the skill to wield it.
Fiendfyre is rightfully considered one of the most potent weapons in a witch or wizard's arsenal. While the Unforgivable curses are arguably more powerful, letting Fiendfyre loose in the right situation is far more effective, as long as one is utterly unconcerned with collateral damage of any kind. It is labeled as Dark magic because of its destructive potential, and because most who use it couldn't care less that collateral damage is a side effect of the spell. Those who have trained in using the spell, a small but non-trivial number, gain a measure of control over the dangerous fire they summon, able to manipulate the spell to reduce the damage they do to things not intended to be their targets. Reduce, but not eliminate, as even practiced Fiendfyre runs a bit on the wild side.
Those who simply attempt the spell, whether out of ignorance or desperation, often find themselves consumed by it.
The last thing on Ginny Weasley's mind at that moment was control. She needed something powerful, and she knew she would only have one chance to cast anything offensive before a hail of curses rained down upon her. She needed something a little wild, a little dangerous, and absolutely devastating. Never mind the fact that she had almost no practice with elemental spells of any kind, as they weren't really part of the general Hogwarts curriculum. Never mind that most magicals don't even attempt to cast area of effect spells that they are only familiar with by name. Right now, the only thing that mattered was surviving the swarm of Death Eaters invading Hogwarts, and if survival wasn't possible, taking as many of them with her as she could would have to do.
She cast. Contrary to popular belief, there is no actual incantation for Fiendfyre. It is merely a manifestation of one's magic, and when the flames began pouring out of Ginny's wand, it manifested into a giant lioness, one that set upon the Death Eaters with a ferocity only matched at that moment by its caster. At that moment, Ginny's normally brown eyes had turned fiery red, not that anyone was looking at her when a flaming lion was attacked them instead.
The first dozen fell easily, consumed, fueling the flames. As the lioness progressed, Ginny followed, feeding more and more magic into her spell. Friend and foe alike ran, fearing the powerful flames that chased all comers. As the forces the Death Eaters had brought to battle retreated towards the Forbidden Forest, so too did Ginny and her lioness, though her progress could hardly be called retreat.
A large swath of trees was cut down as Ginny chased her prey. Deeper and deeper into the Forest she walked, until she reached a clearing where the Dark Lord had made camp. As she approached, lioness still stalking ahead of her, she heard clapping. Even as his forces shied away from the flaming animal, Voldemort made no move to run from the beast. The man once known as Tom Riddle did not impress easily, but the sheer force of magic and will in front of him had certainly made an impression.
"Quite a display, young one. It seems you are very powerful indeed, to have sustained such an animal as this for so long. I do wonder, though, just how much you have spent on it, and how much more your magic can take. For while you are blind to the strain you have endured, I can see in your face that you won't be able to keep it up much longer." He smiled a cold, cruel smile.
Ginny, enraged that the Dark Lord thought so little of her, pushed even more magic into her fire, consuming a further few Death Eaters that hadn't gotten far enough away. But then, the first signs that she was losing control of the spell began appearing. Her wand hand, which had been pleasantly warm so far, was slowly burning where it touched her wand. Flakes of her skin began peeling up, falling dead to the ground. Her lioness, which had stalked proudly throughout, was beginning to shrink slightly, while also taking on a more snake-like visage. She had won the battle with her foes, but she was beginning to lose the battle for herself.
She made to drop her wand, not caring that the most feared Dark Lord of the age stood mere meters away, but the wood had seared her skin, attaching itself to her, making it impossible to easily remove. The realization of what she had done was beginning to wash over her now, and fear was plainly visible on her face. She tried everything she could think of to cancel her spell, but all the magic further fed the beast, as her control slipped further and further. When it finally broke, the exhaustion drove her to her knees, and she watched as her once proud lioness turned into a king cobra, and came under the control of the Dark Lord.
"And now you see, my dear, the dangers of giving yourself over to a spell. As long as you can feed it, it will be loyal to you, as any wild animal would, but when the food runs out, so does the loyalty. Kill the girl."
Ginny looked up, as the snake reared back to strike.