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Chances are that it was a candidate of a few dagger skins where those not chosen will become Gem Store skins.
Side Note: While I think the legendary rifle HMS Divinity is a neat looking rifle skin, the Arcane Marksman rifle was probably a legendary rifle finalist and didn't make the cut, hence became a gem store skin, which fully fleshed out, would have been a more desirable weapon for me to obtain in-game had it been the Legendary they chose.
