Remus Lupin transforms into a werewolf

Remus Lupin, the new Defense Against the Dark Arts teacher, had kept a devastating secret throughout the school year. He was a werewolf, a condition he had carried since childhood after being bitten. The full moon triggered a painful and involuntary transformation into a mindless, dangerous wolf, a fact he had concealed from his students and most of the staff. Only Albus Dumbledore and the other teachers knew, and Severus Snape had been brewing the Wolfsbane Potion to allow Lupin to retain his human mind during the change. The potion, however, had to be taken regularly, and on the night of the confrontation in the Shrieking Shack, Lupin had forgotten it in the chaos of the evening.

The Transformation Under the Moon

As the group—Harry, Ron, Hermione, Sirius Black, the bound Peter Pettigrew, and the unconscious Snape—made their way back across the darkened grounds toward Hogwarts, a cloud shifted, bathing them in moonlight. The effect on Lupin was immediate and horrifying. His body began to convulse and reshape itself: his head lengthened, his shoulders hunched, and hair sprouted visibly across his face and hands, which curled into clawed paws. The man vanished, replaced by a snarling werewolf that snapped its long jaws. The transformation was so sudden and complete that Sirius Black, who had been leading the way, could only shout for the others to run. The werewolf, now a fully formed monster, wrenched itself free of the manacle that had chained it to Pettigrew.

The Battle and the Escape

Sirius Black, himself an unregistered Animagus capable of transforming into a large black dog, immediately shifted forms to meet the threat. The enormous, bearlike dog bounded forward and seized the werewolf about the neck, pulling it away from Ron and Pettigrew. The two animals locked in a brutal fight, jaws and claws ripping at each other. In the chaos, Peter Pettigrew seized the opportunity to escape. He dived for Lupin's dropped wand, stunned Ron, and transformed back into his rat form, fleeing into the darkness. The werewolf, now free of the immediate conflict, took flight, galloping into the Forbidden Forest. Sirius Black, bleeding from gashes across his muzzle and back, gave chase but was soon overwhelmed by the arrival of a hundred Dementors, who cornered him by the lake.

Consequences and Revelation

Lupin's transformation had catastrophic consequences. It allowed Peter Pettigrew to escape, preventing the immediate exoneration of Sirius Black. The next morning, Snape, furious at the night's events, revealed Lupin's lycanthropy to the entire Slytherin house at breakfast. The news spread quickly, and Lupin, knowing that parents would not want a werewolf teaching their children, resigned from his post. He packed his belongings, including the Marauder's Map, which he returned to Harry. In his final conversation with Harry, Lupin confirmed that he was one of the map's creators, Moony, and that Harry's father, James Potter, had been Prongs. He left Hogwarts, his departure a direct result of the transformation that had exposed his deepest secret and ended his brief, successful tenure as the best Defense Against the Dark Arts teacher the students had ever had.