Meanwhile, back at the school, Sirius was rushing as quickly as possible to make his way to the headmaster's office, searching for any signs of life along the way that may be able to help. He skidded around a corner, nearly to said office, when he ran into one of the few people who were more likely to side with the Death Eaters in a battle.
"Out a little late, aren't you, Black? And where might your other friends be, out in the dark? It wouldn't be a good night to wander far." It was the sneer stretched across Snape's face, even more so than his words, that tipped Sirius off. Snape had known.
While Sirius was aware that time was of the essence, he felt a sudden fury overtake him at the idea that somehow, (though he admittedly wasn't sure quite how) Snape had discovered the secret entrance and revealed it to the Death Eaters. That now they had lost their favorite passage into Hogsmeade, and that somehow, Marauder secrets had been spilled.
It didn't help that Snape happened to be standing in the way of his continued path towards the Headmaster's office, and that experience with his own family had told Sirius quite clearly how unwise it was to turn one's back on an angry Slytherin.
He didn't consider the Gryffindor at his back until he was unconscious, with Peter, shaking and pale, staring morosely at his once-friend.
"Well, thank you, Pettigrew," Severus snarled, annoyed at the boy who had ended a perfectly good fight early and interrupted his own rising place in the ranks of Death Eaters.
He never noticed Pettigrew's wand raising, curse on his lips, until it was too late for him as well.
Peter Pettigrew and Gilderoy Lockhart had very little in common, in the grand scheme of things. Lockhart was influential in life, whereas Pettigrew was only known after death. Lockhart was good-looking, whereas Pettigrew was a short, fat child who grew only to become a short, fat adult. But both were abnormally atrocious at defensive spells, ridiculously cowardly, and knew, better than almost any other spell, how to Obliviate someone.
Snape never knew what hit him.
Harry, meanwhile, had much less time than Pettigrew to make a choice of what to do in his current situation. The options, to him at least, were clear, and followed basic instincts to their fullest: flight, or fight.
And as Sirius had previously pointed out, Harry's chances of avoid curses shot at his back while running down the narrow passageway back to Hogwarts were quite slim.
Intellect, or what remained of it in spite of the panic that threatened to take over Harry's mind, told him to fight. And instinct told him how, using the same curse that had never, in all his time learning, using, and even (to the extent that he had in the DA) teaching it, had never let him down.
"Expelliarmus!" He hissed, adrenaline running through his veins as he saw the two men fly through the air in front of him to land firm against the wall.
Coincidentally, hitting one's head against a large, thick brick wall after flying towards it from across the room at a decent speed has much the same effect as the Obliviate charm preformed miles away up at the castle by Peter at that moment.
Harry watched them attempt to feebly stir, the Death Eater whose name he did not know moaning in pain. Quickly but quietly, Harry sunk back into the trapdoor beneath him, shocked that it had actually worked out so well that the Death Eaters hadn't remembered seeing him, and were unaware now of the trapdoors existence at all.
It was then that it occurred to him that maybe Dumbledore really was correct in believing that no matter what he tried to do, fate would end up doing what was necessary to continue time on it's natural path.
Harry knew then what he had to do. He didn't like it, but he would do it, and then fight even harder to find a way to get back home.
Harry strode confidently down the halls, somehow aware that this was what needed to happen for things to return to their proper place. He was afraid, more than anything else, of what methods fate would use to make sure that things happened correctly if he didn't set about fixing things himself.
That didn't mean he wasn't going to enjoy it a little first though. After all, if he was going to have to erase Sirius's memory, the least he could do was tell Sirius everything first, explain everything that had happened.
Unfortunately, both for Peter and for Harry, fate was not willing to leave it's outcome in one single young wizard's hands. As such, it was Peter that Harry found in the halls, squatted down nervously over Sirius's slack face.
It didn't matter that Peter appeared to be checking on Sirius, or that perhaps at that moment Peter was regretting his decision to join the Dark Lord more than ever before. Harry saw red.
"You just can't leave well enough alone, can you, Peter? How'd you get both him and Snape? From behind?" He hissed, furious to find the round, sweating boy attempting to raise his wand. Before he could even wrap his slippery fingers around his wand fully in the proper position to cast the Obliviate charm, Harry already had his own wand to Peter's nose.
"I know all about you, Peter. I know all about what you're willing to do, and it makes me absolutely sick that I have to let you do it." Peter blinked, confused, and then tensed, petrified with a basic first year charm that he should have been able to block, but couldn't.
And as Peter listened to Harry moan and rage about how unfair his life was, not making any sense in the slightest and babbling nonsense about fate and destiny and such rot, Peter felt his own hatred for the boy before him spread further than ever before, further than it would likely ever progress for any other human being. He'd been wondering, just moments before when he'd seen Sirius's unconscious, familiar face, if he'd done the right thing, becoming a Death Eater. He'd considered outing himself to his friends, even, just to try to keep them safe.
But outing himself to his friends would mean outing more of himself to Harry. Would give Harry just a little bit more of a foothold into their group, just a little bit more leverage with the others that Peter frankly didn't feel Harry deserved.
In later years, when Lily gave birth and James announced that his child was named "Harry," Peter felt a disgust he couldn't explain. While the memories of Harry shouting at him, cursing at him, and paralyzing him were gone, the hate inside Peter remained, festering and rotting inside of him.
And there was little Harry could have done to prevent that hate from boiling over into Peter betraying the Potters anyway, because when it came down to it, fate had plans for Harry Potter, and those plans were too intense, too important, for one mere trip through time to disrupt them.