I wrote this years ago, and I think the idea is sound. Sadly it quickly devolved into farce, and I could never really revive it. Anyone who wants to run with this idea, please do.

Harry Potter and the Good Omens

The acromantulas were everywhere.

"Diffindo! Diffindo!"

Harry didn't have time to look back and see if his friend's efforts were productive, he was too busy carving a path forward for them to run back to the school.

"Defoliatio! Come on, Hermione!"

Hermione ran forward again; "Cobblers, they're circling around in front of us! Arania exumai!"

"Bloody hell, they don't like that… I think they prefer it when Ron cuts their legs off."

"Diffindo! Diffindo!"

"I wish he wouldn't go into a berserker rage whenever he sees acromantulas."

….

"I realize it's in an a hard to see area, Harry, but do you recognize this symbol?" Dumbledore held up a parchment with an occult symbol, drawn in great detail, in the middle.

Harry took the sheet and studied the drawing for a short time. "I'm confused, sir. I can see this perfectly, though I admit I don't recognize it."

"Ah, forgive an old man, I misspoke. As you say, this drawing is perfectly visible, but the one I was referring to-"

"Oh! You don't mean that do you Professor?" Hermione interrupted from over Harry's shoulder.

Dumbledore cleared his throat.

"Leave it to Hermione to recognize it," chuckled Ron, "Here, give us a butchers then. Oh! I hardly think the Headmaster means that."

Dumbledore cleared his throat again.

"Would someone please tell me what is going on?" Harry asked pointedly.

"Apparently your friends have recognized your birthmark, Harry, though I shan't ask how."

"Oh heavens, if we had a galleon for every adventure we ended up stranded, naked, miles from civilization… well, we could retire," Ron laughed.

"You can't retire on 27 and a half galleons, Ron, and the Isle of Wight counts as civilization- well, technically…"

"It's not always the Isle of Wight…" Ron mumbled.

Hermione continued, "Anyway, if we hadn't gotten over body modesty pretty quickly, we'd never have gotten anything done." She paused. "Come to think of it, we'd be dead."

"Yes, yes, that's all well and jolly good, but you seem to be confused. I don't have a flipping birthmark!" Harry gesticulated wildly.

Ron snickered, "Yes you do, on your arse."

"Don't say arse in front of the Headmaster!"

Dumbledore rolled his eyes. "It's not as if I've never heard it before, Miss Granger."

"Why didn't one of you mention that I had some sort of demonic sigil on my bum before?"

"Well, it's a birthmark Harry, it doesn't have nearly as much resolution. And it's only tan on white. But it's definitely the right shape…"

Ron nodded. "Yeah, the outline is unmistakable. Anyway, you and I keep quiet about Hermione's… Um…"

"My what, Ronald?"

"Charming disposition and brilliant wit?"

"Humph. I'll get it out of you later. Right now, I want to know why Harry has an occult symbol."

"On his arse."

"I'd shut up, Ron, you're in enough trouble already," Harry sniped.

"If you're finished, children…" Dumbledore said in a textbook schoolmaster's voice.

"Sorry sir."

"Right. Well, to make a long story short, that birthmark proves that Harry is the son of Asmodeus."

The office was silent for a length of time, with nothing but the quiet hum of phoenix song that Fawkes made, even while simply breathing.

"Who's she?" Harry finally asked.

"Um, Satan, I'm pretty sure," Hermione answered, "What 'proof' leads you to think-"

"I don't think Satan is a she," Ron commented.

"Neither do I, although if anyone could get away with being called a name like Asmodeus, I guess it's the Devil," Harry replied.

"Both of you shut up and focus! Could you be less laconic, Professor?"

"Certainly. You see, as you know, Harry, you were adopted by James and Lilly Potter. In fact, I helped place you there, after I rescued you from… well, I'm getting ahead of myself. Your biological parents are- were- Alistair and Wanda Crowley."

Hermione gasped.

"They're the famous demonologists that had a mysterious disaster 15 years or so ago, right?" Ron asked.

"15 years, 4 months and 17 days, to be precise," Dumbledore agreed.

Harry blinked, "Tempus." The time and date appeared, floating in front of him. He mumbled a few moments, then audibly sighed and said, "Plus about 3 hours and um, 20 some minutes, I suppose?"

"Very good Harry. Yes, at the date, time, and location of your birth… something happened. I've done an immense amount of research, something I think you three can appreciate, and as far as I can tell, Asmodeus imbued you with a shard of his power as you were born. I was one of the first to get on the scene, the Crowleys being dear friends of mine, and the only thing left amongst the wreckage of the main building of the estate was you. Lemon Sherbet?"

The three teens nodded mechanically as they processed the information. Dumbledore handed each of his charges a yellow candy, which they accepted, and robotically sucked on. Dumbledore shrugged, smiled, and popped one in his mouth before continuing. "I had only gotten to you minutes ahead of the cult. Thankfully the Chattering Order of Saint Beryl is more of a support-order… They weren't terribly organized… The three of you could have dealt with them… Anyway, after I apprehended them, I interrogated them to find out what they were up to. Apparently the plan was to have you raised by a horrible family, the Durzels or something, and they would drive you to evil and insanity, whereupon when you grew up, you would start the Apocalypse."

"Why are you so calm? I'm the Anti-Christ! Why am I so calm? Am I going to grow horns?" Harry asked wide-eyed, still sucking the sour candy.

"You three are calm, because I gave you my calming-potion enhanced Lemon Sherbets. I'm calm because there's no immediate danger, and as long as we take precautions, there shouldn't be much in the future, either. As to horns… hmm, you look like you've finished puberty. You should be safe."

"Madame Pomfrey's charts have him putting on another half inch over the next year," Hermione disagreed, sucking her Sherbet. "He's a late bloomer."

"'Mione has to keep tabs on who can fit under the disappearing cloak," Ronald explained, dreamily. "Thanks for giving that to Harry, by the way. Can I have another Sherbet?"

"And to know how much extra capacity I can put on a Tenser's Floating Disk while we ride it. For some reason, treasure is almost always heavy, and always a bugger to carry, pardon my French sir," she added, still monotone.

"I'm going to have half-inch horns?" Harry asked, confused.

Dumbledore frowned, those had been the Sherbets from the upper-right hand drawer, right? "I was jesting. You won't grow any horns." Dumbledore looked back to his Sherbet sanctuaries- the bottom-right drawer sat open, mocking his carelessness. Albus ran his wand through the air in a practiced pattern, and the four of them suddenly spat out the sweets, only to see them dissolve into mist.

"Right, I think everyone's… calm enough," Dumbledore said.

The three students sat with mild frowns now, as the Sherbets disappeared. Finally Harry spoke. "Is that why I created all of that Fiend-Fyre, sir?"

"Almost certainly. You were also able to control it, to a degree. That's… impressive, in a way. But then you've always been magically gifted, haven't you?"

"I suppose. But… Okay, the thing is…" Harry paused, "I don't want to bring about the Apocalypse…" He snorted suddenly. "You don't say that everyday, do you?"

"Only once or twice in my life, Harry. And I'm glad to hear it. From what I understand, the Apocalypse would not be pleasant."

"That's not all there is to it though, right?" Hermione asked. "'Come to my office, Lemon Sherbet, Harry is the spawn of Satan, don't destroy the world, oh, by the way, watch out for that nasty Fiend-Fyre, it'll get ya. Off to bed, now,'?"

"Respectfully, sir," Ron added.

Dumbledore's eyes twinkled as he grinned. "No; there is, of course, more to it. On your 16th birthday, Harry, your powers will manifest more fully. That will be… interesting. However, that is just the beginning. When you manifest your birth-right, it will alert the Powers-That-Be: both Heaven and Hell will eventually realize what I've done, and quite a few very powerful beings will be quite annoyed at the ruining of their plans. I can probably weather it, but some of them may lash out at you in their anger. If you survive the initial reaction, then you will need to be wary of becoming a pawn in the great celestial game."

Hermione huffed. "Jeez, how very coming-of-age, this-boy-is-now-a-man. I suppose he has to deflower a virgin after pulling a sword from a stone, shooting an arrow through a row of hoops, and becoming mayor of London."

"An arrow through a row of hoo-? Oh! That's what Quidditch is about!" Ron exclaimed loudly. "Why didn't anybody tell me it was all a sex metaphor?"

Dumbledore had an expression on his face as if he couldn't tell whether to laugh or be annoyed. "Miss Granger, Master Weasley, I'll admit that we haven't spent a great deal of time together during your studies here at Hogwarts, but if we can extrapolate from this meeting, you both have a powerful talent."

"… Th- thank you, sir!"

"To turn serious conversations into knob gags," Dumbledore finished.

"They used to be such nice kids… I expect I've corrupted them somehow," Harry said, laying the back of his hand against his brow, melodramatically.

"Don't you start. Anyway, I'll be sure to pass your literary criticisms on to Satan, maybe next time he'll have a daughter whom manifests her powers after an aromatic sip of tea brings back a lifetime of memories of hell."

"Oh God, don't get her started on Proust," Ron cringed.

"Can we get back to that bit about pawns and celestial games? It doesn't sound very fun. Remember the giant chess board in first year?" Harry asked.

Dumbledore stood up. "So you three are responsible for taking the golden idol of Chicuxlub, are you?"

"It belonged in a museum!" Ron shouted, "… After a modest finder's fee of course."

"That giant boulder was a great trap. If we hadn't been small enough to fit in the corners of the ramp, we'd have been goners!"

The idea: Harry Potter is the son of the Devil, a la Pratchet's Good Omens.

The trio go into the forest to get an albino Crumple-Horned-Snorckack for Luna's Dad's paper (the Moon.)

It goes poorly, and Harry summons fiend-fyre by accident. He's able to control it -barely- and with the help of Dumbledore (who appears), they manage to put it out.

Back in DD's office, DD explains how Harry's parents had been famous demonologists, but apparently something happened that put them in debt to Satan. No one is quite certain what happened, but the Crowley estate practically exploded the night Harry was born. Neither of his parents were found, but DD found Harry in the St. Cecil and All Angels Church of England, being attended to by the Chattering Order of Saint Beryl. Apparently Harry was to have been raised by this awful family - the Dursels or something- who would drive him evil, and then he would bring about the Apocalypse.

DD rescued Harry and placed him with the Potters, who had lost their child to some nastiness involving a dark lord in the middle east.