Shadow Weaver's origin from SHE-RA is probably my favorite "E.C." story to date. It's full of tragedy, pathos, and a flawed, easily relatable protagonist who's unknowingly transformed into a villainess. Best of all, it allows the possibility of Light Spinner being redeemed one day.
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(It's kind of ironic that a comic book based off a
children's cartoon provides better background and motivation for a character's downfall into evil than most adult-centered programs and novels these days.)