: How can I make sure there is only one direct descendant over a thousand years Long story short, a thousand years ago a man killed himself halfway through a ritual, which stopped it in half

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John does the ritual. He has several kids.

Maybe most or all them die before having any children of their own.

Even if they do, heck, they can can all die off, even three or four generations in. It CAN just happen to shake out that way.

Certainly possible. Absolutely. But narratively, there isn't any real reason to go into the hows and whys of the fact that there's just one person left in the world that is his direct descendant, unless it adds something to the story.

I would say, that if you do want to get crunchy with it, or you have to absolutely know the mechanics behind it, you should follow other poster's suggestions and ask another board. Don't mention it's for a story, just ask the question: "Is it possible for a man born 1000 years ago to have just one descendant living, given that the man had 5 [or however many] children? Are there people walking around right now who are, so to speak, genetically unique in this respect? I know things can get exponential with generations and just wanted to see if this was plausible in any way, and how it might work?"

It's an interesting question.

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The family gets religious, of some sort, which states that you can only have one child, and if it is twins the weaker must be killed.

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This is a rather complex answer. Since you did not specify how prevalent magic is, so I'll assume fairly low magic.

If you look for paternal descendants, then having mostly daughters will solve your problem. If the descendant needed can be of either gender, then the BBEG could have been trimming parts of the family over the years. Over centuries, humans spread out across the land. Think back to your own family a thousand years ago... heck even a century ago... Where did they live? What about their own siblings?

Since magic can be factor, perhaps the family have a mark that passes from one generation to the next, perhaps they have a special hair or eye color?

How does the BBEG keep track could provide help. Was he watching over the family from afar? From nearby?

Not a definitive answer for sure, but hopefully some avenues of thought for you to investigate...

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