This story is also available in audio, which you may find on this episode of Wingardcast.
The immortal was a little grey lady in an outfit so outdated that it would have looked more suitable in a Renaissance faire than in the nursing home that would have suited the woman herself. Skirts atwirl, she swished over to her captive with a sigh and touched his face. He was generically handsome, hadn't shaved in a couple of days, and bore a look of utter panic.
"I thought you were a myth," he choked.
"No," she said kindly. "I'm sorry."
"Please let me go."
"No," she said again.
"Why do you do this?" he cried.
"If I didn't," she said, patting his hair with a critical expression, "I'd die."
"But if you do I'll die."
"That's true," she acknowledged. "It's you or me. I don't take people with families, you know. It's just the two of us."
"But I want to live!" he wailed.
"So do I," she cooed. "I think I'll get a haircut..."
"You've had God only knows how many centuries...!"
"Mmm... nine and a half," she said, after a moment's thought.
"Isn't that enough?" he begged.
"Not if you're doing it right," she said with satisfaction, looking up at the ceiling.
"I have a full life ahead of me..."
"Me too!" she said. "A fuller one, really. Without me, you'd still die, you realize. It would take a bit longer, but... if you do it right... you won't be any more interested in dying then. Without you, though, I would miss out on an eternity. Just think! So much to do and see."
"But you have no right."
"I guess we'll agree to disagree, then," she said, sitting down on the little stool before where he was chained up. She smoothed her skirts. "I hope the moths don't get these while they're put away... you haven't the figure for them at all; don't take that as an insult."
"Find someone else," he pleaded. "Someone who doesn't care if they die."
"Now, that would be irresponsible of me," she said severely. "Preying on the mentally ill! I hope you don't think they're less deserving than you."
This statement so baffled him that he left off arguing with her for a short time.
"I think I'm rather nice, considering," she said. "I don't kill people gratuitously. I wait until I'm good and old. I once spent forty years missing a leg, and that was before all the fancy prostheses and laws about handicapped accessibility. I could have taken another body then, but no, I waited until it just wasn't safe any more. This one's going to have a stroke any minute now, and let me tell you, it's not because I don't eat right!"
"But you don't have to kill people at all," he said.
"I do if I want to live. I tried to move into a bird once. Tried a cat, tried a few other things - doesn't work."
"Haven't you tried just not moving into anyone?"
"Can't say it crossed my mind. I'm not suicidal, you know," she says. "The way I see it, this is just self-defense. I mean, if I were to let you go, now, wouldn't you try to kill me? Or, maybe you wouldn't - but say I let you go and then had a heart attack. There's a phone on the wall. It's hooked up. Would you call me an ambulance?" He didn't answer her. "So I could let you let me die and then, eventually, after a good long run, you'd die too. Or I could move in and have - oh, you're probably good for sixty more years! And I can carry on indefinitely."
He sobbed, and she patted his cheek sympathetically. "It's all right, darling. Doesn't hurt a bit," she murmured. "Is there anything you'd liked tidied up for you after I've moved you out? Letters that need sending, charities you want me to send your things to? I'm not going to keep them. That would be stealing."
His head drooped. He muttered a nonprofit's name, and she nodded sagely. "You have my word," she said.
She laid her palms on his skull and closed her eyes.
Half an hour later, the timer on the restraints made a small beep and he was released. He got to his feet, picked up the corpse in front of him, and carried her out to the ready-made grave in the backyard.