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If he weren't as riled as he was, Bellatrix's icy silence would have driven terror into his heart. Despite all of the his insecurities and fears, Regulus was at his heart still a Black - and he would not back down. He didn't want Bellatrix's involvement, and her fury was only solidifying that he had made the right decision. The slight changes in her expression that were only noticeable because of the intensity of his gaze left him vindicated that the information was not for the Dark Lord himself. Whatever way Bellatrix tried to spin this, Regulus knew now that she had been meddling when he had expressly asked her not to. He had attempted to put space between them before, but now he needed it desperately. He couldn't afford for her to be messing around in his life.
"I know, Bella. I know. I don't doubt your intentions, nor your devotion. But I have made my boundaries clear, and you are repeatedly crossing them. I want to train with Rabastan. I've asked you to leave me be...my ambitions are mine and mine alone, and I'm comfortable with where I am. I want to serve the Dark Lord, but I also want to get on with my job at the ministry and study magic itself. I have little interest in climbing ranks, I just want to study," he said, mentally building further occlumency walls in his mind in case Bella decided to pry.
Regulus hated using the unforgivables, but avada at least was smooth and painless. He didn't have the stomach for torture, he never had and he never would. On a battle field he would use avada over any other curse, wanting to end the suffering quickly and neatly. At her instruction, Regulus froze, staring at Bellatrix blankly as she finished her tea.
The issue with unforgivables was that you had to mean it, and Regulus never did. With a slight shake to his hands, Reg pointed his wand at his cousin. "Crucio," he said, voice weak. The curse appeared to bounce off Bellatrix harmlessly, so before she could say a word he tried to summon up some of the fury in his veins from their earlier discussion as he cast the curse again.
If her previous silence had been chilling, the utter emptiness that followed his next speech could have frozen hell itself. For a moment, her eyes widened exponentially, her lips pursed and nostrils flaring with an outrage that flowed her veins like a devastating tsunami. He had called her bluff and she did not like it. Of course she hadn't gone to the Dark Lord. He wouldn't care for such a pointless piece of information, but that was besides the issue. She'd still needed the whole picture before deciding what to share with him. As she glowered, she placed both hands onto the arms of her chair and dug her nails into the upholstery. Better that than around her darling cousin's throat. It was a long time before she spoke again. Long for Bellatrix, anyway - at least a minute, in which she had struggled internally with the desire to deny the fear he'd so aptly identified. She never admitted it. Never. Her mask was one of merciless rage and relentless disgust. These things were far easier to acknowledge. They were simple to redirect, channelling her fear and her hurt into something more palatable. Blacks did not show weakness. But her family falling apart made her weak in ways that repulsed her.
"I only want what's best for you and Cissy." She took his words on board, trying her utmost to give him space. But she would be damned if she abandoned him completely. There was only so far that her patient goodwill would stretch. "I only want you to be happy and secure. If your ambition is so lowly, then I cannot change that. But while there is strength in my bones, I will do my best to keep you alive."
Regulus's sighing and flinching did nothing to soothe Bellatrix's concern. Quite the opposite, she was left feeling anxious - a state she most despised. But she was at least relieved to see his killing curse hurtling into a bust of old Corvus Lestrange, even if he'd had to speak it. Raising her fresh cup of tea, she took a deep gulp. "That was good, cousin." Not good enough, but she wasn't going to say it. Even she could see he needed praise. "Now Crucio me. But let me finish my tea first, I need to wash down that horrible ginger snap."
Finishing it fairly quickly, she then pushed the cup aside and held up her hands with a laugh. "Ready when you are. Don't feel bad about it, I don't mind a little pain." Truth be told, she quite enjoyed it. All that carnal monstrosity coursing through her every fibre.... Delicious.