Q: Why do most warlocks take ones with pure spirit energy to experiment on?
A: Because pure spirit energy is easiest to corrupt. The more one practises spirit arts and experiences life, the complicated and harder it becomes to corrupt them the way the warlock wants, so that's why they prefer practitioners of younger age and with purer spirit energy.
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It took me far longer than I had assumed to copy all the scripts written on the wall. Well, I first assumed it would be like regular writing, but when I started, I learned, it was a lot harder to copy a language I don't know. Moreover, I needed to be precise with each mark, or it might get confusing.
I think it was not just the old tongue, but the ancient rune script. Or why else would it be that complicated?