Disclaimer: I own some stuff. Not Harry Potter, or anything in that universe.
This is one of those 'collection of story seeds' things. Figure it's good practice.
Sirius was falling. Arching gracefully, Harry watched the only person he'd ever felt any love for slip away. Remus' arms tightened around his middle, holding him in place.
Something inside him snapped. Harry started struggling to get clear of the wolfman's grip. Stronger than a normal wizard, Remus was able to hold the raven haired boy away from the veil. Taking a deep breath, Harry let the rage inside himself swell and grow. He now only saw red, electricity making his nervous system scream with tension. He felt strength coursing through his body and took full advantage of it without any conscious thought.
Twisting slightly, he grabbed one of Lupin's wrists and peeled the man's grip loose as if he were nothing more than a child wrapped around his waist. Remus' eyes went wide as he realized something was very, very wrong. Letting the fury inside himself direct his magic, Harry felt it roaring through every single cell of his being. His clothes grew tighter with each breath he took, tears leaking out of his eyes. His face was stuck in a rictus of anger and rage. Growling deep in his chest, he felt his shirt give way, ripping apart at the seams as his body continued to transform. In a few short seconds, Harry Potter had gone from a mere slip of a teenage child to something different.
There were many legends that might have related themselves to what was happening to Harry, but the only one that really applied was that of the berserkergang. It had often been said that Harry had inherited his mother's 'red headed fury' as he had been prone to angry outbursts before. This eclipsed them all as the light of a single candle might have been outshone by the light of a thousand white hot suns blazing in the sky.
A thin line of spittle had begun to drip out of the side of Harry's mouth as he turned, very slowly, to face Lucius Malfoy. The white haired man blanched with fear, having some kind of idea of what he was seeing. He pointed his wand at the growling teen and started cursing.
He might as well have been throwing handfuls of peanuts for all the good it did, as Harry moved to attack with such ferocious speed that he hadn't even had to dodge. He simply wasn't present where the curse had been thrown. One heartbeat after the curse had left Malfoy's lips, so too had his life left his body. Harry hit the man at full speed with a mighty forearm, snapping his neck in an instant. As Malfoy fell, Harry kept moving, latching onto the man's robes with both hands and heaved him into another pair of people, sending them crashing to the floor.
Coming to a stop, Harry roared at the top of his lungs. An unholy sound that rattled the walls and made everyone present quake in fear. Remus Lupin had heard animal sounds like that before from some of the wizarding world's mose fearsome beasts. Never before had he heard something quite like this - it was an angry sound, promising a horrifying and painful demise to whomever was foolish enough to have invoked the wrath behind it.
The boy hadn't stopped moving, instead had adjusted his course and was moving around the perimeter of the room with horrifying speed. This broke the Death Eaters out of their stupor as they dropped whomever they'd been holding and started sprinting, pell-mell for the door to the death room. Harry caught them at the door, stomping over them in an obvious attempt to reach Bellatrix Lestrange. Taking two Death Eaters, one in each monstrous hand, the enlarged teen smashed them together with a wet slopping sound. Weasley turned away from the scene to be sick on the floor and Longbottom collapsed to his knees, shaking. Dropping the corpses, he roared again and bounded down the passage in pursuit of the remaining villains.
The order members had collected everyone from the death room, and were now heading out to the main ministry entryway. They weren't moving quickly which was for the best, as Harry wasn't leaving any survivors in his wake. They could hear him engaged in some sort of destructive mayhem up ahead.
The children were all huddled together, behind Lupin and Shacklebolt. As they were about to enter the main ministry foyer, a pale and shaken Albus Dumbledore met the group and stopped them.
"Sweet Merlin, wait," he said as he held himself up with the wall.
Having seen all manner terrible wartime atrocities, Dumbledore should have been the last person to quail in the face of what Potter had become. Some sort of crash sounded from the atrium, followed by some high pitched screaming. Albus cringed terribly, and fumbled around in his robes. "Kingsley, I'm going to create a portkey to the infirmary at Hogwart's. I want all of you to take it. Right now"
Nodding, the auror helped the injured to all get close enough to use the magical item and as they all whisked away, the man wondered what could possibly have been going on to cause Dumbledore such distress.
As the portkey activated, Albus Dumbledore gathered his resolve and turned back to the atrium to see what sort of aid he could render. Walking out into the open again, he arrived just in time to see Harry slamming Lestrange's unconscious body into a wall while gripping an ankle. She was a barely recognizable mess of flesh and wounds, obviously mortally wounded. Harry had blood all over himself, none of it his.
"Harry," said the Headmaster. "Harry Potter. Look at me"
As Harry turned in a crouch to eye the old man, his blood red eyes coming to rest locked with Dumbledore's. He dropped Bellatrix, her corpse hitting the floor with a wet squishing sound. Dumbledore suspected that there wasn't a single bone larger than a matchbook that was still intact. The headmaster observed again the horrific changes that had been wrought upon the Potter boy's body. Likely around seven feet tall, and hundreds of pounds of muscle were all packed onto his colossal frame. The most striking change was the palpable rage pouring off the boy's aura.
"Now I need you to..." Dumbledore was interrupted by the sound of someone apparating into the atrium. Harry whirled and lunged at the interloper with a feral cry, spittle hanging loosely from his lips.
Voldemort had arrived in the atrium to join the battle against the Order of the Phoenix members. What he didn't expect to see was a berserk and enlarged Harry Potter tearing at him with furious purpose. His eyes wide, he started casting and cursing as fast as he was able. Stunners had no effect, same with cutting curses. By the time the third curse left his lips, Tom Riddle realized he might have made a mistake he couldn't recover from as one large hand grabbed him by the face and launched him bodily into a wall.
Albus Dumbledore had seen a great many things in his life. Now he was witnessing something that completely defied his ability to comprehend it. Wisely, he chose to observe from a place of cover. Lord Voldemort was casting curses as fast as he was capable of - both direct damage spells as well as transfiguring debris and flinging elemental effects at Harry. All to almost no effect at all.
The floos in the atrium started lighting up, and it was all Dumbledore could to to pull everyone out of harm's way and cast a 'blocking' spell to close them all off before anyone else got pulled into the crossfire.
Voldemort was panicking. The monster moved far too fast for him to catch a killing curse, seemed immune to most every bit of elemental magic he was capable of casting. This fight wasn't going well and he knew it. He had to get out of the ministry and figure out how to deal with this new threat. His musings cost him precious seconds and he failed to dodge out of the way quickly enough to keep Potter from getting a grip on one of his ankles.
"Let go of m..." Lord Voldemort didn't have time to finish before he was bodily slammed into the statue in the fountain. Potter was using the Dark Lord as a cricket bat and attempting to bludgeon the statue into stone chips. Agony was all Voldemort knew, his body snapping and breaking under the horrible forces being directed to it. He felt one of the boy's feet stomping his ribs as he was shoved under the fountain water's surface. "This must be the power I know not," he mused in a daze as he succumbed to blessed unconsciousness.
A week later, at Hogwart's, Harry found himself sitting in the Headmaster's office feeling a little nervous.
"Well, Harry. I must say, you've certainly managed an inventive method of dealing with the Dark Lord." Dumbledore was chuckling outwardly, as a cold fear gripped his stomach. He was sitting in the presence of one of the fabled true berserkers and it wouldn't do to get the boy angry. Albus knew he wouldn't like Harry when he was angry.
"I'm just glad we all got back ok and nobody was permanently hurt." Harry was really a little worried. His past experiences with the wizarding world made it clear that he was always under public scrutiny and he was currently in one of the 'negative public opinion' moments.
Nodding, Albus popped a Lemon Sherbet into his mouth and considered what to do next. It's not like he could punish Harry for his actions, for he'd been quite justified in his rage - nobody was denying that. It's just the ferocity of his methods that had the public in complete fear of him. The ministry had been able to strip Tom Riddle of his magic, basically locking him into the broken and battered body Harry had all but destroyed. Horcruxes or not, Riddle wasn't going anywhere. The Dark Lord's reign had been decisively ended in just one night, all thanks to the efforts of one black haired child and his unholy rage.