Thorn was always in Yor's possession. He referred to it as his rose before his corruption. Yor saw the "thorns" on Thorn as petals like that of a rose. After his corruption, all he saw was thorns.
English
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Actually I think the weapon originally looked a lot more beautiful in the traditional sense of the word. We have an artifact called Dregden Yors Rose and it's just a flower, it's possible that originally the weapon was covered in these petals and then over time as it was corrupted it became the jagged frame we know today.
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There is no traditional sense as beauty is always in the eye of the beholder. Yor could have viewed what we call Thorn as a thing of beauty before he was corrupted.