What it does
Subterranean Assault doesn't exist as a real Magic keyword—it's not a recognized ability word, keyword action, or mechanic in any official set or rules document. If you encountered it on a card or in a discussion, it may be a typo, a fan-made/custom card, a misremembered name, or a Commander house rule.
If you're thinking of a real ability, a few similar-sounding mechanics might fit: Burrow/Burrowing (a Bloomburrow-era concept) doesn't exist either, but Saddle and creatures with "can't be blocked" effects come up often. There's also Subterfuge-style hidden-information mechanics in non-Magic games.
For an accurate ruling, double-check the exact card name on Gatherer or Scryfall. If it's a custom card for your playgroup, the table should agree on its precise wording and timing before the game—undefined keywords cause the most disputes in casual Commander.
Iconic cards with Subterranean Assault
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