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As the title says, when and what made Voldemort look the way he does (pale, snake-like etc) as opposed to how we've seen him in his earlier years as Tom Riddle? Is it just a side effect of his unnatural resurrection? Before he died attacking Harry as a baby did he look like a normal guy?

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It almost certainly was his Horcruxes.

Dumbledore says that that the Dark Lord’s transforming looks were only explicable by his creation of several Horcruxes, and the mutilation of his soul that creating them caused.

“I, who have gone further than anybody along the path that leads to immortality.” That was what you told me he said. “Further than anybody.” And I thought I knew what that meant, though the Death Eaters did not. He was referring to his Horcruxes, Horcruxes in the plural, Harry, which I do not believe any other wizard has ever had. Yet it fitted: Lord Voldemort had seemed to grow less human with the passing years, and the transformation he had undergone seemed to me to be only explicable if his soul was mutilated beyond the realms of what we might call usual evil …
- Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)

Since Dumbledore was basically always right about the Dark Lord, this is almost certainly the explanation. Additionally, it fits with what we know of his appearance through the years - it changed gradually, not all at once. At eleven, he was described as quite handsome, and in no way unnatural or disfigured.

“There was no trace of the Gaunts in Tom Riddle’s face. Merope had got her dying wish: he was his handsome father in miniature, tall for eleven years old, dark-haired and pale.”
- Harry Potter and the Half-Blood Prince, Chapter 13 (The Secret Riddle)

As a teen, when he went to kill the Gaunts he remained handsome, with no noticeable changes to his appearance.

“The door creaked open. There on the threshold, holding an old-fashioned lamp, stood a boy Harry recognised at once: tall, pale, dark-haired and handsome – the teenage Voldemort.”
- Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)

There’s still no mention of any changes in his appearance after he kills the Gaunts - nothing unusual in it is described.

“Half a dozen boys were sitting around Slughorn, all on harder or lower seats than his, and all in their mid-teens. Harry recognised Riddle at once. His was the most handsome face and he looked the most relaxed of all the boys. His right hand lay negligently upon the arm of his chair; with a jolt, Harry saw that he was wearing Marvolo’s gold and black ring; he had already killed his father.”
- Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)

After graduating Hogwarts, Tom Riddle was still handsome but he was starting to change somewhat by then.

“The house-elf returned within minutes, followed by a tall young man Harry had no difficulty whatsoever in recognising as Voldemort. He was plainly dressed in a black suit; his hair was a little longer than it had been at school and his cheeks were hollowed, but all of this suited him: he looked more handsome than ever.”
- Harry Potter and the Half-Blood Prince, Chapter 20 (Lord Voldemort’s Request)

His eyes began to gleam red, like they’d eventually become.

“I wonder whether you know what it is, Tom? Pick it up, have a good look!’ whispered Hepzibah, and Voldemort stretched out a long-fingered hand and lifted the cup by one handle out of its snug silken wrappings. Harry thought he saw a red gleam in his dark eyes.”
- Harry Potter and the Half-Blood Prince, Chapter 20 (Lord Voldemort’s Request)

The next time we see him it’s ten years from then.

“Ten years separate Hokey’s memory and this one, ten years during which we can only guess at what Lord Voldemort was doing …”
- Harry Potter and the Half-Blood Prince, Chapter 20 (Lord Voldemort’s Request)

By then, he’s already changed quite a bit from how he looked as Tom Riddle, but he didn’t look like he finally would as yet.

“Voldemort had entered the room. His features were not those Harry had seen emerge from the great stone cauldron almost two years before; they were not as snakelike, the eyes were not yet scarlet, the face not yet masklike, and yet he was no longer handsome Tom Riddle. It was as though his features had been burned and blurred; they were waxy and oddly distorted, and the whites of the eyes now had a permanently bloody look, though the pupils were not yet the slits that Harry knew they would become. He was wearing a long black cloak and his face was as pale as the snow glistening on his shoulders.”
- Harry Potter and the Half-Blood Prince, Chapter 20 (Lord Voldemort’s Request)

He isn’t described explicitly the night he kills the Potters (though we know from Harry’s nightmares of that night that it was the same as when he was returned to a body) but we see that his appearance is unusual enough to scare a Muggle child.

“Nice costume, Mister!’

He saw the small boy’s smile falter as he ran near enough to see beneath the hood of the cloak, saw the fear cloud his painted face: then the child turned and ran away … beneath the robe he fingered the handle of his wand … one simple movement and the child would never reach his mother … but unnecessary, quite unnecessary …”
- Harry Potter and the Deathly Hallows, Chapter 17 (Bathilda’s Secret)

When he was returned to a body, he looked the same as when he killed the Potters, since Harry describes him as the face that haunted his nightmares - which almost certainly are the nightmares he’d been having about the night his parents died.

“The thin man stepped out of the cauldron, staring at Harry … and Harry stared back into the face that had haunted his nightmares for three years.”
- Harry Potter and the Goblet of Fire, Chapter 32 (Flesh, Blood and Bone)

He’s described more explicitly when he returns to a body, and from this description we learn both what he looked like when he killed the Potters and after his return to a body.

“Voldemort looked away from Harry, and began examining his own body. His hands were like large, pale spiders; his long white fingers caressed his own chest, his arms, his face; the red eyes, whose pupils were slits, like a cat’s, gleamed still more brightly through the darkness. He held up his hands, and flexed the fingers, his expression rapt and exultant.”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)

The changes happened gradually, and weren’t caused by his return to a body since he returned looking like he did before.

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