They aren't mine. Oh, and I couldn't resist… yes, the imp is inspired by Disc disorganizers. A very silly little thing that jumped into my head right before Christmas.
As a seventh year, she told a young Eileen Prince that her Muggle lover would never marry her and would leave her with a dozen children to take care of alone, disgraced and disowned by her family. That had been a bit farfetched, but it was only her first time.
Two years after graduating Hogwarts, she told Dumbledore that the way to unlimited power was to succeed Dippet as headmaster of Hogwarts School. By that time, she'd mastered chanting in a trancelike state, and, when combined with her ability to roll her eyes back in her head at will (a talent she had placed a taboo on, forcing anyone who mentioned it to speak in Sanskrit instead of English), it made for quite convincing prophecies.
When she was getting a bit short on funds for sherry, and was getting paid for it anyway, she claimed to Dumbledore (overheard by Eileen Prince's only child) that the child born at the end of July would have the power to defeat He-Who-Shall-Not-Be-Named. Had she not been hankering for the divination post at Hogwarts, she would have found a better venue to broadcast the fake prophecy, but as it was, she figured it would get out soon enough.
It was true that she constantly made silly little predictions of doom, and Dumbledore did not believe these, but she had him "look, line, & linker"—a Muggle expression she was rather proud of knowing—on the big fakes, where it counted.
Eileen Prince, fortunately for Trelawney's sake, was not even supposed to be in Divination and took an altogether more sensible view of prophecy, allowing that foresight was at most like the quantum paradox (either view the result or else the mechanism, never both) or possibly the box of Schrödinger's cat. She was very skeptical of the whole mess, besides.
In any case, Slughorn's goal for which he had asked Trelawney to falsify a prophecy (in exchange for a few points on her potions project), was not at all fulfilled. Part of the thwarting was in Sibyl's getting too carried away with the prediction, and a lot of it was carried in a misunderstanding that a "Calvin" they had heard Eileen talking about was her boyfriend instead of her cousin.
Through curious twists of fate, Eileen did indeed fall in love with a Muggle, and their relationship was troubled… but Tobias Snape married her, loved her at ultimatum although he quit showing it when he found out she was a witch, Ephineus (her father), already semi-estranged from his family, did not disown his daughter or her half-blood son, and that half-blood son (with a more prophetic name than Trelawney's fictions) was her only child.
Dumbledore had taken Sibyl's word for it, and had played the ace up his sleeve to become Headmaster after Armando Dippet. Had he been thinking more sensibly, or possibly if he'd had a Slytherin's acutely penetrating view of politics, he would have realized that becoming Headmaster was a dead end street off the road to power. It did provide him with a certain power… but it automatically limited what he would be able to do and opportunities for advancement. He did not groom a successor because he did not want competition—he was unwilling to play in such an arena—and so he could not escape the post as Headmaster.
In that case, Sibyl's purpose had been to make Dumbledore owe her a favor and to impress upon the influential man her powers of prediction (false though they were). She succeeded in her purpose, and, somewhat ironically, when combined with her next prediction, created a great deal of power for one Albus Brian Wulfric Perceval Dumbledore.
With regards to the monumental prophecy that would intensify the war, it was only a cruel joke of fate on the false prophetess that made Alice Longbottom, a woman Sibyl was actually quite fond of, deliver her son two weeks early, resulting in their being targeted alongside the Potters. Dumbledore certainly believed it, and prepared for the war based mostly upon that. Quite whether Severus Snape, who delivered the false prophecy believed it or not could not be told by the man himself. He did not discern that the prediction was a malevolent plot against the Potters; then again, he had taken to totally ignoring the Potters since Lily Evans Potter had broken off their friendship, and probably wouldn't recognize that his former friend was due at the end of July. Beyond that, he was used to delivering lies and half-truths to a flash-tempered, cruel master he was somewhat reluctant to be serving in the first place.
On the other hand, fate delivered the pain of this false prophecy (despite allowing blood to be shed against her price) mostly on the head of Snape (who was forced into a double life) instead of punishing Trelawney for faking a prophecy that would inevitably result in the extermination of the Potter clan.
That Harry James Potter was "the one with the power to vanquish the dark lord, born as the seventh month dies" with "power the dark lord knows not" was complete poppycock framed in grandioso language by Sibyl, who was paid a handsome sum by a group of several Slytherin families (who weren't actually associated with the Death Eaters) to make the Potter family the primary target of Voldemort's anger.
Many lives had been lost by that bit of ridiculous nonsense, including, most recently, the departure of Sirius Black, godfather of Harry Potter, through the Veil in the department of mysteries. Being the site of his beloved godfather's death meant the room grieved Harry sorely, and why he was supposed to be here with a Madame Bones—an important individual in the ministry—and his most-hated Potions professor, who was reading aloud what seemed to be a nonsense poem, he hadn't quite caught.
"…You see, the purpose of this room had been most foolishly lost," Madame Bones was saying. Harry's gaze lighted upon an iridescent black globe, swirling with silver and specks of green, growing like a bubble at the end of one of the shelves.
"When I was finally able to recover the mechanism by which this room captures bits of dialogue, most amazingly, it turns out that it's a room of things that people think might be important to the future of wizarding Britain that the Ministry can't fathom the meaning of—"
"Do you mean the prophecy's not true?" Harry queried in astonishment, as the globe popped, much like the bubble it had resembled.
He heard a very small bell ring, and a tinny voice announced "Charles Lutwidge Dodgeson, 1860s"
"No-o; well, not verifiably true. Actually, we think Trelawney might be a fraud. After all, for a short time, Professor Snape was able to make the room think that a published nonsense poem was actually important in some mysterious way to the future of wizarding Britain. The algorithm is actually quite fallible, and it often doesn't sort out original arrangements, even if they are completely false."
"What?" asked Harry, who hadn't been entirely able to follow the explanation. "And why is Professor Snape here?"
"Professor Snape is here because he is very good at Occlumency and Legilimancy. And what I was saying, Mr. Potter, is that the prophecy is probably of little to no actual significance, because it's not verifiable."
"On the contrary, Madame, it is very important, because I assure you that once He-Who-Shall-Not-Be-Named and Dumbledore are both convinced of something, you're going to be pulling teeth to get the population to disagree with it." the Professor retorted, sotto voce.
"I think we're more sensible than that."
"Really. Unless the dunderheads that I have to teach potions to are not indeed a typical sample, I fear the wizarding public of Britain is nowhere near as intelligent as you make them out to be." the Potions Master retorted in typical sardonic fashion.
"What was that bell?" asked Harry.
In response, Madame Bones muttered something that sounded like gibberish, and a little compartment popped out of the ceiling, containing a creature something like a miniature house-elf, which was seated, rather cramped, over a huge tome whilst its other hand rested on a bell ringer.
"That is how the Ministry determines prophecies?" asked Snape.
"Well…yes," she admitted.
Harry looked around the room, looked at the creature again, and then burst out laughing for the first time since Sirius's death. "It's all wrong! I'm not the deliverance of the wizarding world after all!" he exclaimed as if he were going insane. It was just all so unexpected and strange, after all!
"Potter, while you may not be the deliverance of the wizarding world, being rid of the prophecy does not rid us of He-who-shall-not-be-named." The professor was really being a spoilsport, now.
"Harry! This is important!" exclaimed Hermione.
"But we did really just find out that the prophecy is a fraud, right?" Harry asked, blinking out of his half-sleep to find a frazzled Hermione and a grumpy Slytherin Head looking at him, among the others in the meeting.
"Yes, and whilst it is commendable to study for your OWLs, Mr. Potter, late last night was hardly the proper time." Professor McGonagall scolded, giving him a frightful glare over her spectacles.
"As soon as this meeting is over, you will go straight to bed, and you will have detention tomorrow evening in my office for staying up so late." The Headmaster put in, pulling violently at two lemon drops that were stuck together.
"Really?" he couldn't help asking, this behavior being so uncharacteristic of Dumbledore.
"I'm just jesting, my boy." Dumbledore reassured him, as the sweets gave way and his hand banged into the bookcase.
Snape, however, apparently couldn't resist a dig at him. "Five points from Gryffindor for failure to pay proper attention."
The ending did not come in the same wave of inspiration that provided the first part, so it may not be so good… and I actually turned it a surprise direction because I seemed to be writing Harry so out of character. They may be a bit out of character… but it is AU and rather off-kilter.