Augustus Rookwood, gen. When they take him there, he doesn't panic. The dementors can't touch him. There is nothing he regrets.
A Life's Work
Augustus Rookwood doesn't snivel, boast, shout or lie like the other war criminals on the stand. Instead, he just faces the Council of Magical Law as he used to face the heads of sections and of the Departments, pleasantly blank and taking whatever he is given. He conducts himself with dignity as he is sentenced to Azkaban for passing "valued Ministry information" – could he really help discussing things with other Department Heads? - and causing the deaths of (as Barty Crouch says with relish) "countless people."
Augustus replies with complete sincerity, "I don't think that's a very legal way to approach multiple homicide."
When they take him there, he doesn't panic. The dementors can't touch him. There is nothing he regrets.
He manages to sleep with this assurance, but when he wakes, he feels his dead wife's lips on his. Lucretia. His eyes open immediately, but there is nothing there; a spare memory the dementors dredged up from endless years of pointless marriage.
The dementors pass by and the air turns to ice around him, but he is a logical man. He picks splinters from his bed and scratches equations into the floor, documenting each day on the wall. A month in, he starts shouting for someone to come in, because – by now he's run out of space on the floor and is writing on the other wall – he's found something new, oh god, give him a wand and he can ward the whole of London, give him parchment and a quill and he can discover all the equations that make up the world itself.
Four months in he's tested his theory ten, twenty times, the equation burnt into the back of his eyelids. He shouts at the others to shut up, shut up, he's a genius and he's working, shut up.
One year in he's run out of space. He has nothing to write with. He makes himself bleed – oh yes, he knows how to draw blood – and the colour of it blinds him after years of grey and brown. Reverentially, he scrawls bright red on the walls.
Two years in Azkaban, he is unable to puncture his calloused fingertips any further.
Four years, he relives in painful, vivid detail his son's funeral, his son, murdered by Rufus Scrimgeour, no less than three days before his own arrest.
A month later he dreams of pushing his knives one by one into Lucretia's body, carving her mercilessly just like so many years ago, but not a drop of blood soaks into her lavish robes. There is a begging look on her face.
Michael. We'll name him Michael, she says in a firm tone, her lips loosely tracing the words.
He has done nothing wrong.
He draws blood from his palms and scrawls, over and over until the wall is red, I have done nothing wrong.
The dementors close in on him. The walls close in on him. He knows the secrets of the universe, there is nothing for him to fear.
He raises his head when the murmur of voices fades. He rises to his feet and notes that the air is no longer thick with the presence of dementors, and he hesitantly touches the door. Eventually he opens it. He hears the wild laugh of Bellatrix Lestrange, and manages to smirk.
Before leaving, Augustus glances back at his cell, at his life's work, and mouths every last line. Then he walks to greet the others, his true family, and leaves the prison, cleansed of his past and baptised in his suffering for his Lord.