What it does
I'm not familiar with a Magic: The Gathering keyword or ability called "Brand-new Sky." This doesn't match any official keyword, keyword action, or ability word that I know of from any MTG set or supplemental product.
It's possible this is a misremembered name, a fan-made card, a card from an unofficial set, or something very recently released that I'm not aware of. If you can share the actual card text or where you encountered it, I'd be happy to help explain how it works.
If you meant a different keyword—like Skulk, Flying, Cascade, or something with "Sky" in a card name—let me know the correct term and I'll give you the full rundown on what it does, the common rules pitfalls, and how it plays out in Commander.
Iconic cards with Brand-new Sky
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