The woman's nails and lipstick are the red of blood, and Bellatrix Lestrange has been following her for over an hour. She's stalking the insolent witch who upset Narcissa so by causing Lucius to get into a common bar brawl in order to show what happens to fools who get above their station. (The woman certainly wasn't from any pureblood line Bellatrix has ever heard of, and she's furious at the idea of this mudblood mixing with normal people as if she had the right.) Bellatrix is growing tired of waiting for her to go home, and is considering dealing with her now, and waiting to punish her family later, when the woman turns around with a smile that could cut glass.
"Bellatrix Lestrange," the woman says, "You'll do wonderful things if you keep listening to me."
"What gives you the right to speak to me at all, let alone assume I would listen to you?" Bellatrix demands, wand raised. "I am a servant of Lord Voldemort and you are nothing."
The woman throws her head back and laughs, teeth sharp in the moonlight. "Who do you think whispered plans to him when he slept as a child?" she asks. "Who do you think is behind every killing curse you cast, every dark mark you create?"
"You have no right to speak of the Dark Lord that way," she hisses. Than, louder, "Avada Kedavra".
Green light streaks from her wand and hits the woman straight in her chest, but she doesn't crumple to the ground. She grabs Bellatrix's wrist in her hand, forcing her wand to the ground.
"You haven't figured out who I am yet, have you?" she asks, shoving Bellatrix up against the wall of the dark alley. She leans close, whispering in her ear, "I am older than your Dark Lord, older than your pureblood dynasties. I sang cradle-songs to Voldemort, to Grindlewald, to Herpo the Foul. Every action you take on behalf of Voldemort serves my purposes."
"Very poetic," Bellatrix spits, "But complete nonsense. Why don't you try telling me who you really are?"
"I'm War," the woman says, "And while this world exists, so will I." She kisses Bellatrix then, hard and bruising, and her mouth tastes like pomegranates and ashes.