He knew who she was bfore she announced herself, before she even reachd the landing; he had followed her on the way up, felt her approach from the bottom of the steps. He had his ways, but but there was no need for them with her: she was still far too young, and far too inexperienced, to know how to shield her thoughts, or even realise she should. So her mind ran loose, a wild and unguarded little thing, bright with raw want and honest determination, spilling thoughts and feelings with every step. She was so certain he would grant her wish that she seemed incapable of any feeling but pride.
Seemed.
With a mind as opening as hers - almost begging him to dig deeper, so willing to tell him her deepest, darkest secrets - it took barely a minute to look where even she could not. Buried deep beneath the pride - so deep he did not know if she even knew it was there; if she did, she would prove an impeccable occlumens once trained up a little - he found a small, childlike fear that whispered, again and again: Please don’t send me away.
And he found himself, for the first time in many years, quite surprised; none of his followers had ever feared being dismissed by him; they were far more likely to fear being summoned. All but his oldest friends avoided him whenever they weren’t needed, skirting around him like wary animals. So why was she—a new one, a young one, someone who had only ever known him as Lord Voldemort, who had no reason to seek him out—climbing those stairs alone and uninvited, with no fear to speak of besides "don’t send me away"?



It intrigued him. So when she finally knocked, 
She startled; she had not expected it to open so quickly, before she had even finished her first knock. But she tried to hide it, a flicker of satisfaction already taking its place. He had not said no. He had not sent her away. She crossed the room quickly until she stood a foot from him, then lowered herself to one knee and waited. And the longer he had her wait, the tenser she became. Bot with the oily fear he had come to expect from the men who served him, but with something sharper, almost reckless—a willingness to face whatever answer awaited her simply to have an answer at all.
That, too, intrigued him. It also irritated him. For all the noise in her mind—wanting, hoping, bracing—there was not a single coherent thought about what she wanted. She was too consumed by the desire to ask to actually think the request itself. He would have to make her say it aloud.
"Bellatrix," he acknowledged her. "You were not summoned."
"No, My Lord." Still she did not look up, her head bowed and her face hidden beneath her hair. Still, even now, her mind was a whirl of anticipation with no shape to it. No clarity. No thought he could seize.
“My Lord, I have come to … ask something of you."
"Then ask."
"Mark me."
He could not keep himself from smiling. "Ah," he said. "So very bold ... I must admit, I did wonder how long it would take you to ask."
For a heartbeat, he simply looked at her.

Ah. So it was that.

He had expected this desire—of course he had. Ambition clung to her like a second skin; she burned for significance, for belonging, for him. He had known she would ask eventually. But not now. Not alone. Not with this raw, unguarded sincerity. Not with her mind a storm of hope so loud it drowned out the thought itself.

He could not keep himself from smiling.