Disclaimer: JKR owns Harry Potter. The Italicized portion at the beginning was taken directly from The Order of the Phoenix. Nothing is mine except for the idea.
Only one couple were still battling, apparently unaware of the new arrival. Harry saw Sirius duck Bellatrix's jet of red light: He was laughing at her. "Come on, you can do better than that!" he yelled, his voice echoing around the cavernous room.
The second jet of light hit him squarely on the chest.
The laughter had not quite died from his face, but his eyes widened in shock.
Harry released Neville, though he was unaware of doing so. He was jumping down the steps again, pulling out his wand, as Dumbledore turned to the dais too.
It seemed to take Sirius an age to fall. His body curved in a graceful arc as he sank backward through the ragged veil hanging from the arch. . . .
And Harry saw the look of mingled fear and surprise on his godfather's wasted, once-handsome face as he fell through the ancient doorway and disappeared behind the veil, which fluttered for a moment as though in a high wind and then fell back into place.
Harry heard Bellatrix Lestrange's triumphant scream, but knew it meant nothing — Sirius had only just fallen through the archway, he would reappear from the other side any second. . . .
But Sirius did not reappear.
And in that moment something changed.
A powerful wind tore through the chamber, causing members of the Order and Death Eaters alike to pause. This was not the result of a spell, or accidental magic, but something much older and much more powerful.
The momentary lull in the fighting was replaced by the sound of Bellatrix Lestrange's mad cackling. All eyes turned to her as she gleefully ran about the room.
"I killed Sirius Black!" Bellatrix sang while pointing her wand at Harry. This pulled everyone from their stupor as spells began firing from each side.
Harry could feel the anger coursing through him. His godfather was the closest thing to a family he had ever known, and he had been ripped away from him. Bellowing in rage, Harry pointed his wand at Bellatrix and screamed "Crucio!" Bellatrix, seeing the curse rushing toward her, moved out of the way and fired back in kind.
But nothing happened.
More confused than anything, Bellatrix tried again. "Crucio!"
But still, nothing.
By this point, the fighting had once again stopped. Both groups watched as the mad witch tried spell after spell, to no avail. The wind was back, pushing Bellatrix's hair wildly about her, emphasizing the ever-growing apprehension and fury on her face.
"What did you do?" She screamed, pointing at Harry. "Is little baby Potter so scared of me that he can't even fight me like a proper wizard?"
"It wasn't him."
Bellatrix spun around, a sneer on her face as she looked down on her niece. "What did you say, blood-traitor?"
"I'm a blood-traitor? Because my mother married a muggle-born?" Nymphadora Tonks scoffed. "At least I didn't kill the head of my family."
The blood drained from Bellatrix's face. "What?"
"Sirius Black was the Lord Black. You owed him your allegiance." Tonks said. "And then, to make matters worse, you tried to attack the new head."
"You're lying. Potter isn't the Lord Black. Sirius wasn't even the Lord Black. He was disowned." Bellatrix said confidently.
"You know, my dear Aunt, for all that you spout off about this pureblood nonsense, you think you'd be better versed in your own family's history. Sirius was re-instated. Great Aunt Walburga was so proud when he was tossed into Azkaban. Felt that he finally came around." Tonks rolled her eyes. "And did you really think that Sirius would just let the title be passed on to you crazy tossers? He changed his will the first chance he could. Harry is the Lord Potter-Black."
All eyes turned towards the Boy-Who-Lived.
Harry felt more confused than ever. He knew that Sirius had wanted him to come live with him, but he never realized how much the man considered him family. To pass on the title of Lord Black, Harry thought from where he stood slumped against a pillar, was enormous. Yet in his heart, Harry felt happy. His godfather, who was cruelly taken away from him time after time, had still found a way to make them family. Sirius was Lord Black, and now he was Lord Black. It was like they shared something permanent.
This subconscious acceptance was more than enough for Ancient Magic to take hold.
The wind in the chamber picked up, howling in the ears of everyone in the room. And as she locked eyes with Harry, Bellatrix felt fear creep up her spine. His vivid green eyes were glowing with an other-worldly shine, as waves of power rolled off of him. The entire room watched as magic took hold of Harry and changed him from Harry Potter to the Lord Potter-Black.
Though he looked basically the same, it was easy to tell that some sort of old magic had transformed Harry. His features had a more aristocratic look to them, much like his godfather's had, and his hair was no longer unruly, but longer and seemed to lie more flat. The most noticeable change was his stature. Harry seemed to have grown several inches, and stood with a confidence that few had seen in the boy.
"Bellatrix Lestrange." Harry began in a regal voice, laced with power. "You have committed many crimes in your life. These could all be forgiven in time. But to raise your wand against your lord? This will not go unpunished."
Bellatrix, feeling more and more like a caged animal, lashed out, raising her wand to show the Potter boy exactly who she was, when her wand was suddenly ripped from her grip. It flew into Harry's outstretched hand.
"Did you hear nothing I just said?" He asked menacingly. "I am your lord. You killed the previous. Do you really think that I would allow you to try to do so again?"
"You are not my lord!" She hissed.
The entire chamber seemed to freeze as Harry laughed. A deep, resonant laugh that reverberated around the room, filling the Death Eaters with dread.
"Are you referring to the filthy half-blood you call Voldemort?" He asked, his hand darting out to grab her left forearm, twisting it about so that the Dark Mark was visible. "He's more wraith than he is human."
Inspecting the tattoo, Harry's eyes darkened as he recalled his purpose.
"You subjugate yourself before a self-styled dark lord. You place this madman above your family." With every word he said, Harry's voice gained another edge, as though every Black that came before her was judging Bellatrix and finding her wanting. "Call him."
"What?"
"Call your chosen Lord here so I may speak with him."
"You can't just summon the Dark Lord at will." Bellatrix said, shrinking under his piercing gaze.
Tightening his grip on her forearm, Harry stepped closer to Bellatrix, bringing his lips up to her ear. "You will call him. Or I will kill you where you stand."
A tell-tale pop of apparition came from beyond him. The room sprang into action as Lord Voldemort appeared in the middle of the chamber, Order members firing upon Death Eaters, as Death Eaters kept one eye on the battle and the other on their volatile master.
"Harry Potter." Voldemort began. "Do you so badly wish-"
"Silence." Lord Potter-Black cut the Dark Lord off. "I want to show you something."
Harry pulled Bellatrix forward, tossing her to the ground in front of the Dark Lord.
"You know this woman?" He asked.
Voldemort glared down at her.
"Master, please, I'm sorry, he's done something to me." She pleaded.
"Quiet!"
The command came from both Harry and Voldemort, who glared at each other.
"Tom Riddle," Harry started, noticing the way Voldemort's eyes flashed at his given name. "This woman, Bellatrix Lestrange, by rights, owes her loyalty to the House Black. Instead, she, and many others in the family, have sworn fealty to you. This displeases me."
"And what will you do about it, Harry? You cannot hope to win against me. You don't have the strength." Voldemort said.
"The Noble and Most Ancient House of Black has declared you an enemy of the House." Harry stated simply. "We will hunt you and your forces until no trace of you remains on this earth. We will destroy you so thoroughly there won't be even a hint of spirit to banish to hell. You will cease to exist. And those who defy their House," Harry looked down at Bellatrix "Will be punished."
The room looked on in shock as Harry raised his hand, lifting Bellatrix into the air. A sickly orange light surrounded her as Harry began to speak.
"Bellatrix Lestrange nee Black. You have betrayed your family. You place this maniac so far above your House you would see its end, and have tried to do so by killing your Lord. You are not worthy of your title."
There was a great flash of light.
"Of your name."
Bellatrix began to scream.
"Or of your magic."
The ever-present wind whipped about, encompassing Bellatrix where she floated above the room. Voldemort apparated away, terrified, as Harry's aura began to visibly pulse, showing how powerful he truly was.
"The Noble and Most Ancient House of Black dissolves your marriage with Rodolphus Lestrange. The Noble and Most Ancient House of Black strips you of your magic." Harry stated with a calm ferocity, each sentence punctuated with a flash of magic. "The Noble and Most Ancient House of Black casts you out. From this day on, you will be Bellatrix No-Name. You will find no shelter or refuge under our roofs. You have no family. We turn our backs on you."
With this, Bellatrix dropped to the ground, sobbing. She couldn't feel her magic anymore. She knew what the Lord Potter-Black had done and what it meant for her. She collapsed completely.
The remaining Death Eaters, seeing their Master flee and one of their most powerful rendered useless, took this as a sign to leave while they still had the chance.
"You!" Harry turned, pointing at Lucius Malfoy. "Stay where you are."
His fellow Death Eaters had no qualms about leaving the eldest Malfoy behind, and quickly fled the scene while they had the chance.
"What do you want, Potter?" Malfoy spat. "I do not answer to you."
"You will watch your tongue, Malfoy. My family was Ancient and Noble while yours were still peasants in France." Harry said, eyes flashing. "I truly don't give a damn what you do. But if you do not cease your interactions with the Dark Lord, the House of Black will terminate Narcissa's marriage into House Malfoy. We will reclaim her, her son, and her dowry, leaving you penniless and heirless. And then we will kill you. Now leave."
Malfoy didn't need to be told twice and apparated away, his mind heavy with the threat the Lord Potter-Black put over his head.
Harry walked past the still sobbing Bellatrix to where Neville sat.
"My apologies, Neville. As Heir Apparent of House Longbottom, by rights, Bellatrix should have been yours to deal with. I offer her now, as recompense for the sins against your parents." He said, helping Neville up.
"Just leave her for the Aurors, Lord Potter-Black. House Longbottom was happy to cede this." Neville replied.
Harry nodded, then sat down as the effects of using so much magic finally caught up with him. Remus came and sat next to him, offering him a piece of chocolate.
"You know, Harry" Lupin began. "Sirius would be proud. It's just like him to have one big prank left."
Harry smiled, his heart aching, but realizing that Remus was right. It was just like Sirius to go out with a bang.
"Hey Tonks!" He called.
"Wotcher, Harry. What can I do for you, oh great and noble Lord?" She asked smiling.
"You and your mum want back in the family?" He asked.
"What? Are you serious?"
"No, that was my godfather." Harry grinned.
"Oy, watch it, you swot." She replied. "You'd really reinstate us?"
"Yeah, if you want."
Seeing her nod, Harry began to speak. "The Noble and Most Ancient House of Black is pleased to welcome back our family, Andromeda Tonks nee Black, her husband Ted, and their daughter Nymphadora." A flash of magic signified the change.
Tonks grinned and threw her arms around Harry. "You have no idea what this is gonna mean for mum. Thanks, Harry."
"You're welcome, Nymphadora." He smirked.
"Don't call me Nymphadora."
"I'm the Lord Potter-Black of the Noble and Most Ancient House of Black, and I am your lord. I'll call you what I want, Nymphie."
Tonks groaned, Remus chuckled, Harry smiled, and a squad of Aurors finally burst through the door.
A/N: I've had this idea in my head for a while now. I always thought it was odd how Bellatrix was able to kill Sirius with no weird, magical repercussions. So this was just my take on that!