The revelation of Peter's secret happens due to one of his many blunders. A simple mistake – which ends up costing the Dark Lord his secret agent, and is just the stroke of luck the Order needs to win the war.
Peter receives the Mark just before he returns to Hogwarts for his seventh year. He isn't blind – he can see that the winds of victory are blowing towards the Dark Lord. He isn't about to be caught on the wrong side – he values his survival more than anything else.
As he leaves that first meeting, the pain of getting the Mark burning through his veins, the Dark Lord gives him a warning in his cold, dispassionate voice.
'Keep a glamour on my Mark at all times,' he says. 'The only reason you are alive is because you are extremely useful to me, little rat. My own spy inside Dumbledore's little band of misfits. I have no wish to lose you to Azkaban simply because your forgetfulness caused you Mark to be noticed by one of your simpering friends. Trust me; you would not like it in there.'
That night, Peter dreams of a cold island in the middle of the sea, the Dark Lord's high voice and the scent of fear.
That last year at Hogwarts, Peter manages to keep his Mark hidden. As expected, he joins the Order of the Phoenix with his friends as soon as they leave school. Their close friendship with James Potter ensures that Remus, Sirius and Peter are included in the Order's core group – Charlus and Dorea Potter are still remembered by many of the older members with fondness and respect, which transfers over to their son and, by default, the friends and girlfriend he spends all his time with.
It is then that Peter proves his worth to the Dark Lord, feeding him with information on Order members and missions. Slowly, the number of fighters on the side of the light falls, killed because of Peter's information. And nobody ever suspects that the quiet, mousey boy is behind it all.
His latest mission is to sow distrust within the Marauders. The Dark Lord sees them as one of his greatest threats, the core that holds the Order together.
Luckily for him, there are ready-made ways for Peter to carry out his mission. Sirius is a Black, Remus a werewolf. This shouldn't be too difficult for him at all.
In another world, Peter carries out his mission to perfection. But in this one, he makes a small, but crucial mistake. He grows too prideful over his position near the Dark Lord. He forgets the glamour for his Mark.
At Sirius and Remus' little flat, he begins to pour poison into Sirius' ears. 'He's a werewolf,' Peter whispers. 'It's in his nature to return to the alpha who created him. He's still in touch with Greyback. He doesn't support us at all.'
He does not notice Sirius staring at his forearm in horror. Nor does he see the silvery werewolf burst out from Sirius' wand. He notices nothing – until it is too late.
The revelation of Peter's true loyalties comes as a rude shock to the Marauders. They have never doubted the allegiance of their best friend, trusting in the bonds forged at Hogwarts.
Sirius' revelation of his attempts to make Sirius distrust Remus makes it all the worse.
As the rest of the Order untangles the web of information Peter reveals at his trial – including members killed due to Peter's actions as a spy and a list of Death Eaters that contain several shocking names – the remaining Marauders spend their time in a haze of disbelief.
Peter – no, Pettigrew now – causes the arrest of dozens of previously unknown Death Eaters. Lucius Malfoy escapes by buying his way out in exchange for even more names. As the trials start, Voldemort's army is thrust into a state of chaos. In the confusion, Regulus Black makes his way to Sirius, Barty Crouch Jr by his side, a locket clutched in his hand, bearing news of the horrifying truth he has discovered. It is he who tells them about Voldemort's attempts at obtaining true immortality. It is Regulus' defection that finally calms Sirius' anger, happy that at least one of his family members saw the error of his ways.
Around them the Order stirs into action, looking for the Horcruxes hidden in precious heirlooms. Here, Dumbledore's knowledge of Voldemort's past helps them greatly.
It takes a year, but by the time Lily announces she is pregnant, the only part of Tom Riddle's soul left on the earth is the piece inside Voldemort himself. Even with the Horcruxes and most of the Death Eaters taken care of, Voldemort still poses a grave danger to the Order and the wizarding world. As Lily's seventh month of pregnancy comes around, however, He-Who-Must-Not-Be-Named is but a memory in the minds of the wizards and witches of Britain, who are now occupied with rebuilding what Voldemort had destroyed.
As the years go by, James and Lily have two more children. Sirius and Remus adopt two, a girl and a boy, as well as taking in Draco, his cousin Narcissa's child, who, along with her husband, was killed by vigilantes a few months after the child's birth. Time moves on, and the first of the next generation children leave for Hogwarts. Through it all, the Marauders never forget their missing friend.
Peter dies within a decade of imprisonment in the fortress of Azkaban, the Dementors sapping him of every part of the boy he had once been. His old friends organize the funeral. Even though he is remembered as a traitor, they cannot forget the boy they once knew.
No one ever realises the impact of exposure. In another world, an orphaned boy is left on the doorstep of his magic-hating aunt and uncle's house.
But in this one, a simple mistake changes the course of fate.
For:
The Endless Possibility Challenge, Era Challenge, Marauder Era. Peter is caught being a Death Eater before the Potters go into hiding.
The If You Dare Challenge, Prompt 150. Tick Tock Goes The Clock
Musical Terms Challenge, Bisbigliando
Latin Challenge, Comes
HP Potions Competition, Antidote To Veritaserum
Quidditch Positions Competition, Referee
Something Like It Challenge, Opportunity or something like it
Seven Deadly Sins Competition, Pride