A/N: I will get back to my other fics, I promise. After NaNoWriMo, though.
Disclaimer: I own nothing.
The first time Tom Riddle read about a Patronus, he scoffed at the idea of love and happiness protecting someone and continued his Defense homework.
The first time Tom tried to cast a Patronus, not even a wisp of light escaped his wand, and he proclaimed the spell utterly useless. Of course a spell based on love and happiness wouldn't work.
The first time Tom came across a Dementor was in his search for Horcruxes, and he desperately tried to cast the foolish spell. The monster crept closer and closer, and it was only the timely arrival of Ministry officials that saved him.
When Tom Riddle died, and Lord Voldemort was born, he gave only a mocking thought to that spell made of positive emotions.
As Lord Voldemort surveyed the hoard of Dementors gathered from Azkaban below him, he noticed a few in front were edging closer. He sneered. They would not be as stupid as to attack him, surely? Him, who freed them, him, who would let them feast?
Apparently, some were that stupid. They drifted ever closer, and he raised his wand threateningly. They continued to approach, and perhaps they could sense he could not cast that one spell that repelled them. He felt all the rage and hate that festered inside him all his life build in his chest. His grip on his wand tightened, and his mouth twisted into an ugly sneer as he spoke the incantation that had forever eluded him.
"Expecto Patronum!"
It was to everyone's surprise, including Lord Voldemort and his followers (wasn't he supposed to be loveless, heartless?) that a fully formed silver shape burst from the tip of his wand. It resembled an oversized pepper pot, with strange adornments sticking out at seemingly random intervals. It swiveled its—head?—to look at Lord Voldemort, and then zeroed in on the closest Dementor.
"EX-TER-MIN-ATE!"
Silver light shot out of one of the—arms?—protruding from its lower body. The moment it hit the Dementor, the repulsive creature burst into silver flames and disintegrated. The magic folk stood in shock as the seemingly invincible beings met their end. The Patronus destroyed the remaining Dementors nearest to it, glowed bright white, and vanished.
Silence reigned for a moment, before the Dementors bowed (or their version of it at least) and awaited orders.
Lord Voldemort grinned viciously. He should have known that no spell could be sustained by love and happiness. That was just ridiculous.
A/N 2: This came to me at lunch, when the lovely Capricornicopia pointed out a t-shirt to me that had "My Patronus is a TARDIS" printed on it (so getting that now), and I thought, "Who would have a Dalek Patronus?" Seeing as MoldyShorts couldn't have tortured or glared the Dementors into service, and there was nothing stopping them from soul-sucking MoldyShorts and his groupies too, this was born.