So what is The Power He Knows Not? It all comes down to this.
Harry breathed heavily as he faced his mortal enemy on the field of battle. His vision was filled with the corpses of those already fallen, most by Voldemort's indiscriminate killing spells. Voldemort seemed to care little about complicated magic, merely sending killing curse after killing curse into the panicked crowd, downing more than a few of his own hapless minions who failed to get out of the way.
Voldemort stood arrogantly with a faint, hideous smirk on his malformed face, his wand triumphantly held aloft. He kicked a charred skull away almost casually as he walked closer to Harry, ripping through dead bodies with mere thought alone.
'This is it,' Harry thought mournfully. 'After all this time, all the death I've cheated, all the Horcruxes I may or may not have successfully destroyed, all the killing curses I've dodged... Nothing stands between me and Riddle, nobody left in sight to help... I'm on my own for the first and last time, let's hope the Power He Has Not finally shows its bloody face.'
Voldemort frowned as he stopped walking about ten feet from his foe. "Hmm, seems almost too easy after all this time, Potter. Ah well, can't say I particularly care that you'll be simple to finish off. /Avada Kedavra/." Voldemort incanted the killing curse emotionlessly, catching Harry completely by surprise.
Harry flinched as he waited the last split second it would take to die by the unerring curse, knowing in his heart he would not dodge in time. The green light came closer, and closer, and closer still. Time slowed down and Harry swore he could hear his own steady heartbeat and his blood flow thoughout his body, he thought he might just see his life start to flash before him.
"Nooooooooo!!" A shrill cry startled Harry and Voldemort, and without warning, Ginny Weasley leapt in front of the curse. The curse struck her and she collapsed to the ground, dead instantly. Harry looked up confusedly at Voldemort.
"Tom, I thought you had already killed her."
Voldemort looked thoughtful. "I suppose I must have missed. Well, no matter. /Avada Kedavra/." The killing curse sped toward Harry, who was still shocked by Ginny's sudden appearance and failed to move in time.
But Harry was saved yet again when Ginny's older brother Ron Weasley leapt in front of the curse with a girlish squeal. "I'm the hero!" Ron squeaked out before falling silent, forever.
Now Tom looked annoyed and snarled at Harry. "Potter, I don't know what game you're playing , but I tire of your foolish attempts at survival!" With that, the Dark Lord sent five killing curses at Harry with rapid succession, who prepared to leap aside, a bit more prepared this time.
Of course, it turned out to be unnecessary.
"Not my beloved Harry-pookums!" Hermione yelled in an oddly pleasing baritone, surprising Harry, who had thought their relationship was just platonic. Hermione crumpled to the ground on top of the deceased Weasleys, finally reuniting with her sexually confused friends.
"I hate you Harry, but now you'll remember that I-" Draco's final tirade was cut off while he jabbered incoherently to his old nememis, plowing into the rapdily growing pile of corpses, dead instantly.
"I don't even know you!" Blaise Zabini shouted before succumbing to his inevitable death. Harry nodded at this, he really didn't know Blaise at all. His mother on the other hand...
"Not my darling Blaisey-poo!" A startingly beautiful woman hurled herself at Voldemort, intercepting the next curse.
"Ah, nuts..." Harry mumbled. "I never got a chance to fool around with her."
"Hope I'm not too late to help out, Harry," Neville Longbottom yelled out belatedly, before getting unceremoniously hit by the unblockable curse. Fortunately for him, he was now part of his very first three way with Hermione and Ginny (a long time fantasy), albeit posthumously.
Harry started to chuckle. "Man, looks the the power you have not is a bunch of idiots willing to sacrifice themselves to save me, Voldie."
"Do not mock me, Potter!" Voldemort shrieked furiously. "I have powers you could not hope to comprehend!" With another filthy inventice, Voldemort hurled another twenty rapid killing curses at Harry.
Not worried in the slightest, Harry just stood there and waited.
Sure enough, the next killing curses were caught, in turn, by Seamus Finnegan, Charlie Weasley, Roger Davies, Euan Abercrombie, Wilfred Elphick, Yardley Platt, Owen Cauldwell, and Urquhart Rackharrow, then Arabella Figg, Mundungus Fletcher, Anthony Goldstein, Andrew Kirke, Morag McDougal, Ernie Macmillan, Eloise Midgen, Sally-Anne Perks, Dean Thomas, Orla Quirke, Stan Shunpike, and Zacharias Smith.
"Wow... that was interesting." Harry said in an amused tone. "That last bunch seemed to be alphabetical, but wait... Harry carefully inspected the corpse pile. "Hey, the first bunch of people spell out S-C-R-E-W-Y-O-U!" Harry burst out laughing. "And you having 'powers I could not hope comprehend'? Try using some other curse, Tommy, maybe the Tickling Hex? You aren't having much luck with your old standby."
Sputtering and near blinded with rage, Voldemort charged Harry, letting off several killing curses that had no chance of coming anywhere close to hitting the Boy-Who-Lived. Luckily (or unluckily, depending on your persepective), Harry Potter's beloved teachers leapt in front of the curses.
"We'll save you Harry!" shouted Flitwick triumphantly.
"For Dumbledore!" yelled McGonnagal.
"K'Bloo Fnub!" Hagrid roared.
"Pup! Woof!" Lupin cried, tears flowing copiously down his cheeks.
"Wait for me!" shouted Slughorn in a cowardly fashion.
"No, you don't have to..." Harry started to say. "And... now it's too late. Idiots."
Voldemort, having ignored all this, barreled headfirst into the enormous tower of dead martyrs, knocking himself silly. His mouth was filled with spittle, and he reared back, intent on finishing off his cursed foe. However, the pile of bodies had become too unstable, and the insidious necromantic scents wafting off had finally taken their toll on Voldemort.
The Dark Lord coughed and wheezed, feeling oddly weak. "Can't... breathe... must... cast... killing curse... only... spell... I know..." But it was too late, Voldemort had breathed his foul, disgusting last.
Harry sat down on a nearby corpse to think for a minute.
"Oh no, are we too late?"
Harry looked up, startled, to see the nearly naked forms of Angelina Johnson, Luna Lovegood, Romilda Vane, Cho Chang, Katie Bell, and the Patil twins.
"Girls? What are you doing here? And why are you all in knickers?" Harry probably didn't need to ask this last question, but he was a bit too curious for his own good.
"Oh that," Angelina said dismissively. "We were getting new clothes for the last battle, but I guess we took a bit too long."
"Oh, fudge!" Parvati pouted cutely, crossing her arms under her ample bosom. "Now how are we supposed to help out Harry?"
"Well we could all just play Find the Snogg-flapper with him," Luna said absently.
The other girls looked at Luna blankly, then looked at Harry, then looked back at Luna. Cho turned toward Harry and grinned rakishly. "You know, this time I think I understand what she means."
"Wait a minute," Harry said. "We can't just... I mean what about the legal blah blah bufaiii!" Harry trailed off as he was interrupted by Romilda sticking her finger somewhere unexpected.
"On the contrary, Mr. Potter," the gruff voice of Slughorn wheezed, as the corpulent Professor clambered to the top of a pile of dead Death Eaters. "You will find that your new status as the Chosen One grants certain... privileges."
"Hey, didn't you jump in front of an Avada Kedavra?" Harry accused Slughorn, trying desperately to ignore what Katie was doing to his toes.
Slughorn looked disapprovingly at Harry. "Come now, Mr. Potter, does that sound like me?"
Harry's eyes widened in understanding. "No, but you WOULD jump as though you were trying to sacrifice yourself - and then if I didn't make it, you could always say later that you intended to make it look like you were trying to save mee-ulp!." Harry's words were stopped by Luna quite forcibly sticking her tongue down his throat.
Slughorn nodded, pleased at Harry's accurate analysis. "Precisely, my young friend! But now that the unlikely has occurred, and you alone have survived this conflict with You-Know-Who, I can offer my keen intellect and wisdom to ensure we can rule this country!" Seeing that Harry was no longer listening (although to be fair, his ears were occupied), Slughorn chuckled and leaned back against Tom's corpse, content to merely watch. After all, that was what Slughorn did best.