first wizarding war as a death eater

"The one with the power to vanquish the Dark Lord approaches … Born to those who have thrice defied him, born as the seventh month dies …"

—The part of Sybill Trelawney's prophecy that Snape overheard

Snape with fellow members of the Order Minerva McGonagall, Albus Dumbledore and Alastor Moody.

Added by Patr0nus

Severus Snape eventually joined the ranks of Lord Voldemort's Death Eaters, and was the spy responsible for informing Voldemort about the prophecy foretelling his downfall. In early 1980, Snape eavesdropped on an interview for Divination Professor between Sybill Trelawney and Albus Dumbledore at the Hog's Head Inn. In the course of the interview, Trelawney prophesied that at the end of July, a child would be born who would cause the destruction of Lord Voldemort (which prompted Dumbledore to hire her, partly for her own safety). At that time it was not clear who was meant by the prophecy. Two children of prominent wizarding families were born that year at the end of July: Harry Potter and Neville Longbottom. Dumbledore reported at the Wizengamot trial of Igor Karkaroff that Severus had come to him and explained that he had been the one to overhear the prophecy, and reported it to Voldemort. However, Snape did not hear the entirety of the prophecy, as the barman caught him eavesdropping and threw him out.

Double agent

Severus Snape: "Hide them all, then! Keep her — them — safe. Please!"

Albus Dumbledore: "And what will you give me in return, Severus?"

Severus Snape: "In — in return? Anything."

— Snape switches sides.

Snape casting his Patronus to show Albus his love for Lily Potter didn't die for so time after

Added by Dobby4ever

Severus suffered terrible remorse when Voldemort decided that Harry Potter was the subject of the prophecy, and that Lily was now in danger as a result of his actions. He begged Voldemort to spare her in exchange for the lives of her husband and son. But knowing that he could not leave Lily' safety in question at the hands of someone who could turn back at his word on a mere whim, Snape also approached Albus Dumbledore, who initially thought that Severus came with a message from the Dark Lord for him, to ask for his help in saving Lily; he begged Albus to hide her, along with her husband and yet-to-be-born son if he had to. Dumbledore agreed, but insisted that Severus serve him as a spy among the Death Eaters. Severus secretly defected from the Death Eaters and joined the Order of the Phoenix, under Dumbledore. When Voldemort went to kill the Potters, he did give Lily a chance to step aside, but when she did not, he killed her anyway. When Severus learned of Lily's death, Dumbledore persuaded Snape to help him protect Lily's son, Harry, reasoning that it was the best thing he could do for her.
Snape then dedicated the remainder of his life to protecting Lily Potter's son, Harry Potter, the boy who was often said to strongly resemble James, the very man he detested. Distrusted for his past as a Death Eater by those on Albus Dumbledore's side, and hated by other Death Eaters for living as Dumbledore's stooge for 10 years, Snape continued living on to complete Dumbledore's plan to protect Harry and defeat Voldemort.