The other thing, though, is that BE adds a Destroy effect to an element that doesn't already have one. See also: Pulverizer. Fire only has one in-element way to destroy permanents, everything else requires a good chunk of
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or
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investment.
Imagine facing a tight 30 card monodark (or 35/70 card in the case of arena) with 6 steals and 6 thieves. You wouldn't be able to hang onto anything important unless you had PA.
The card design itself is fine, Darkness is a flavor/style match and I don't think the card is OP on its own. The trouble is when you combine it with more stealage. I'd try to find another element for it. Thieving Trickster Pixiepants
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? Snatching Seagull
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? I know this would require a significant rework of the card, but I honestly think that's the only way this would make it into the game.