Set During the Battle of Hogwarts
Filch stared at the wand in his hand and the body at his feet.
He didn't know how it had happened, only that he'd kicked her; that filthy scum had kicked his cat out of the way like she was just so much rubbish. They came into the castle, breaking things, tracking mud, shattering glass, tearing curtains, scuffing wood, spilling blood on the carpets (he'd never get the stains out), and they dared harm Mrs. Norris!
In that moment, Argus Filch felt, for the first time, something beyond his own impotent rage, felt power rushing through him. Ears rushing, heart pumping madly with the sensation, Filch darted forward with all the speed of an old man who has spent the majority of his life scrubbing floors, but he was fast enough. He seized the invader's wand, let everything he was feeling course wildly through it, and stepped back, panting.
Then it was gone; the glimmer of magic within him had winked out again, but the Death Eater was dead on the floor, not mucking up his castle, not touching his cat.
Filch swept Mrs. Norris into his arms, crooning, "there there, mustn't let that foul man hurt you my dear."