What it does
Hellmasker isn't an official Magic: The Gathering keyword or ability—it doesn't appear on any printed card or in the comprehensive rules. It may be a card name you're thinking of, a custom/homebrew mechanic, or a misremembering of another term.
If you encountered this in a Commander game, double-check the actual card's Oracle text via Gatherer or Scryfall, since the printed wording is what matters for rules interactions. If it's a custom card from a casual or "silver-border" set, you'll need to clarify the intended behavior with your playgroup before relying on it, as homebrew mechanics have no official ruling support.
If you can share the exact card or where you saw "Hellmasker," I can explain the genuine mechanic it's tied to and how it plays in Commander.
Iconic cards with Hellmasker
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