The reason why Joseon was obsessed with killing the Wang family

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Brother Wang Geon, I will prophesy one thing before I die.
There will come a day when this brother will take the lives of your descendants.
Even this guy doesn’t know when that was.

KBStlD
There’s no time anymore
Wang Geon’s brother; I said it clearly
Your descendants will die by your own hands.
Over 500 years; It’s really so lost