She first heard them when she was nine years old. She was playing her sisters, Andromeda and Narcissa, and Narcissa had taken her doll. Suddenly, the drums were beating in her mind and pounding in her blood and before she knew it, Narcissa had dropped the doll and was screaming and her hands were blistered and smoking.
Bellatrix knew that something was wrong, something was terribly wrong, and she couldn't let her parents find out what she'd done. She ordered the house-elf to fix Cissy's hands, and swore her sisters to secrecy, crying penitently and kissing Cissy and promising 'It was an accident, Cissy! I didn't mean too! See, it's all better know – Impy fixed it all up.'
When she started at Hogwarts, she was almost scared to use magic for the first few months – what if she hurt someone again? But she levitated the feathers, and tried to transfigure matches, and noone screamed in pain, and time went on, and the memory of the drums faded and she'd almost forgotten about the drums, til she was seventeen and caught Andromeda kissing a Mudblood in the library. And the fire burnt in her mind again, and she was cursing them both, and Andie was screaming at her, and the librarian came running and had to disarm her and restrain her and send them all to the hospital wing. But this time, she didn't apologise afterwards, because she was still furious, furious that Andie would betray their principles so blatantly.
Two years later, Andie had run off with the Mudblood and left her and their family, and the drums pounded in Bellatrix's head again. She wanted to hunt them down and make the boy suffer, suffer for stealing away her sister, and then she remembered the whispers, the parlour conversations about a wizard who was starting the fight for the pure-blooded regime, a man known as Voldemort…and she thought, what better way to punish them than punish the whole race of Muggles, for daring to produce wizard sons who would steal pure-blooded witches away from their families.
And as the man burnt his symbol into her flesh, the beat pounded through her body (or was it possibly the terrified beat of her heart?), and then she tortured a Muggle family with the rest of them and the drum resounded through her flesh as she laughed at their pain, laughed at the little girls shrieks of pain and terror as her flesh blistered and burned. An apology, a hint of remorse didn't even enter her mind later, and she couldn't forget the drums, because the echo didn't go away, and when she perfected the Cruciatus curse, they rose in an ecstatic crescendo.
Her world collapsed as word trickled in that her Lord had been defeated, defeated by a baby, a baby with a Mudblood mother, and she refused to believe Voldemort was gone, because she hadn't punished the Muggles enough yet, hadn't punished Andie enough, and the drums drove her to search him out, clashing and bashing in her mind as she swept into the Longbottoms house. The beat evened and swelled as she screamed Crucio and she kept going, kept going, and the others followed her, and she didn't stop because it kept the drums sounding right and so the others didn't stop either and then the minds of the Longbottoms snapped and Aurors were apparating in, but she didn't care because Voldemort would come back, come back because the rhythm of the drums told her they had work yet to do, the world was not yet cleansed.
And through all her long years in Azkaban the drums pounded in the back of her brain, and while they protected her from Dementor-borne insanity, they continued to warp her brain in their own manner.
When Voldemort came for her, as she knew he would, the drums beat a staccato, joyous rhythm, and she knew that her world would be right again. And the beat of the drums revolved around his words once more, harmonising with his plans, and yet her mind seemed clear and bright, bright as a burning fire and she'd never felt so powerful.
The drums throbbed as she sent her own cousin through the veil, and clashed horribly as she screamed at Potter in the atrium – screamed of love - because the drums no longer recognised that, and tried to drown it out.
The drums beat without pause for so many long years, driving her on, that, when they stopped for that one infitisemally small moment in between Molly's curse hitting her and her death, she was lost and confused and thought for a moment she was nine and playing with dolls again.