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I think it'd be more likely that he'd murder them both so as to not have any loose ends about him.

But what if he didn't know she had a kid and it was at, like, the very end of his time at Hogwarts but they lost contact and then there was somewhere around 20 (IDK, I don't remember when the first war started) years for the kid to grow up. Then Voldemort when back for the mother but didn't know he had a kid. And then the kid was lucky enough to not be killed in the first war, but he turned to the dark side -- WITHOUT EVEN KNOWING WHO HIS FATHER WAS -- and was killed in the second war????? I MUST GO WRITE THE FANFICTION!

Voldemort wouldn't go back for the mother, he's not sentimental and he considers such things as love to be a weakness. Anyways, even though he could have a kid, I find it very unlikely that he would have one. After all, he never had friends, only followers, much less girlfriends. And I still think he would kill them after he found out they existed.

But wait. I just had an idea. What if Bellatrix was the mother and she just kept it from him???? Is that realistic enough???? I DON'T CARE IF THERE IS A 30-YEAR AGE GAP.

Voldemort would never have sex with Bellatrix. He most likely doesn't see the appeal in having sex and would thereby not hav any. Not to mention he's too busy with his goals of power to distract himself with tedious and petty activities. Besides, Bellatrix's love was an obsessive love, not a true love. And, as you said, there's quite the age difference.

WELL FINE. RUIN MY DREAMS, WHY DON'T YOU. BUT I WILL MAKE IT WORK!!!!!!! SOMEHOW!!!!!! WITH SOME CHARACTER!!!!!! THERE IS A WAY, AND I WILL FIND IT!!!!!!!

I've been debating the idea myself and the only way it seems possible is through accidental conception.

But I don't know how accidental conception would be possible.

Yes. Yes. I might be able to work with that. If I ever stop procrastinating...