Two jets of crimson light flew out of the wand that Black had stolen from Ron. Black's aim was true; the first spell he cast moved erratically, snaking out to Harry's and Hermione's wands, disarming them in a single strike. The second Disarming Charm also struck true, liberating Oleandra from her wand. Black lightly caught all three wands as they flew up into the air.
Even so, Black remained on guard; Harry was accompanied by a Slytherin, he'd recognize those colours anywhere. He had fought countless Death Eaters, many of whom he had known in Hogwarts as Slytherins. Slytherins, Death Eaters. What was the difference? They both fought dirty, and Sirius was certain that Oleandra had allowed him to disarm her.
Inwardly, Oleandra was smirking; she thought that Black had fallen for her bait. Ordinarily, a third year would be helpless without their wand, but Oleandra was one of the few exceptions: she was much more dangerous with her hands free!
Black stepped closer to Harry and Hermione, who were standing over Ron.
"You, Slytherin girl," he said hoarsely. "Walk over to the others; slowly now."
Oleandra obliged him and joined the others at the other end of the room. Now, Black wouldn't have to divide his attention between her and the others.
"I thought you'd come and help your friend," he rasped. He evidently hadn't spoken in a while; he appeared almost unsure of how to open his mouth to say words. His throat had also seen better days, it seemed. "Your father would have done the same for me. Brave of you, not to run for a teacher. I'm grateful… it will—"
It's at that moment, in the middle of his monologue, that Oleandra chose to strike! But unfortunately for her, Black had been on guard for her from the start. Had only the Gryffindors been present, perhaps he'd have been distracted by the presence of his godson. But with Oleandra present, he was acting as if the war was still going on!
He'd even smelled her in his dog form when she'd been floating upwind from him. For what reason had she been standing by instead of helping? Simply smelling her odour had raised his dog form's hackles. He could tell here was something inhuman mixed in with her presence; his animal instincts had told him to get the hell out of there!
"THURSAZ!" she roared, curling her fingers at Black in the shape of the Thorn!
"Protego!" Black growled, summoning an invisible magical shield in front of him!
Oleandra's surprise attack bounced off the magical shield and was reflected directly back at her, sending her crashing through the rotten wooden walls and clear out of the shack with an "oof!" She clearly hadn't been expecting Black to be on guard for her tricks, since he'd never seen her before.
Harry saw red; he started forwards, eager to get his hands around Black's neck and squeeze 'till his eyes popped out… "No, Harry!" Hermione gasped.
"If you want to kill Harry, you'll have to get through us first!" Ron said fiercely, as he desperately tried to stand on his one good leg.
Sirius Black hesitated as memories of his old friends flashed before his eyes. James, Remus… and Peter. This was how it should have been! Why couldn't it have been this way? He'd been prepared to die for his friends, and yet…
"Lie down," Black said quietly to Ron. "You'll damage that leg even more.
"Did you hear me?" Ron said weakly. "You'll have to kill all three of us!"
Brave words, but he wasn't impressing anyone; he was desperately hanging on to Harry to prevent himself from falling over.
"Don't you worry," Black said, flashing a nasty grin. "There will only be one murder tonight."
"And why's that?" Harry spat, trying to free himself from his friends' grips. "Didn't care last time, did you? When you slaughtered all those Muggles to get at Pettigrew… What's the matter, gone soft in Azkaban?"
"Harry!" Hermione whimpered. "Be quiet!"
"HE KILLED MY MUM AND DAD!" Harry roared, as he managed to get loose. To his surprise, Black didn't react nearly as fast as he did when Oleandra had surprise attacked him. To be fair, once his small fists started falling on Black's face, he stopped caring. He just wanted to inflict as much damage as he could on the one who had caused his parents' deaths.
Seeing as it was too late to keep themselves from fighting, Hermione and Ron joined the fray as best they could, throwing kicks at Black's tall and lanky figure as he and Harry rolled around on the floor. Crookshanks also decided to jump in… on the bad guy's side. It scratched and hissed at Harry.
Somehow, they all managed to separate from this tangled mess, with Harry ending up with his wand. Hermione, all scratched and bruised, also scrambled on the floor to recover both her wand and Ron's. Harry pointed his wand straight at Black, who was still on the ground.
"Going to kill me, Harry?" Black whispered. Indeed, how exactly was Harry planning to do such a thing? He hadn't learned any spells that could kill someone; maybe he could perform a Full Body-Bind on Black, then crush his head under a rock while he watched on, helpless?
Meanwhile, Oleandra was seeing stars; quite literally. She'd been forcefully ejected from the Shrieking Shack with enough power to break anyone's neck, and thankfully, Cloak had absorbed most of the impact. When the dizziness abated, white-hot anger flared up within her; this was the third time her own magic had been reflected back at her! The first time, it had been Lord Voldemort in Professor Quirrell's body; the second time, it had been Mafalda, the Heir of Slytherin; and now, it was some stupid mass murderer who looked like a homeless person! The first two she could accept, but him!?
"Never again," Oleandra swore to herself, as she got up to her feet and flew back through the hole she had made. To her surprise, she found that Black had already been handled; Harry and Hermione (who was dual-wielding) were pointing their wands at the empty-handed madman.
"You killed my parents," said Harry, turning his head slightly to acknowledge Oleandra's arrival.
"I don't deny it," Black said quietly. "But that's not the whole story."