What it does
I'm not aware of "Unearthly Power" as an official Magic: The Gathering keyword or ability. It doesn't appear in the comprehensive rules or any keyword glossary I can verify.
It's possible you're thinking of a similarly-named card, ability, or perhaps a term from a custom/unofficial set. A few things that might be the actual reference: Unearth (a graveyard-recursion keyword that returns a creature with haste, then exiles it at end of turn or if it would leave), or specific cards with "power" in their effects.
If you can share the exact card name or where you encountered "Unearthly Power," I can give you an accurate rules breakdown—including any Commander-specific nuances. I'd rather not invent mechanics that could mislead you during a game.
Iconic cards with Unearthly Power
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