In ancient Egypt, Anubis would use a scale to judge whether the soul was qualified to ascend to heaven. Some people even said they knew the weight of the soul: 21 grams.
Everything has weight. If your scale is accurate enough, you can see the soul. In magic, if you can know the weight of any object, it is equivalent to putting an invisible mark on it. From this concept a wild idea has appeared, and a prop with unlimited potential has arrived; this is SOUL.
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