Lucius found out later on (I can't remember if it was HBP or DH) but in COS he doesn't know. I think Lucius was acting on Voldemort's orders? In this book:
Quote:
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"I decided to leave behind a diary, preserving my sixteen-year-old self in its pages, so that one day, with luck, I would be able to lead another in my footsteps, and finish Salazar Slytherin's noble work."
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