What it does
I'm not aware of "Relentless March" as an official Magic: The Gathering keyword or ability. It doesn't appear in the comprehensive rules or any released set that I can verify.
It's possible this is a custom or homebrew mechanic specific to Mythicwyrm, a misremembered name, or confusion with a similar-sounding card or ability. For reference, there are real cards like "Relentless Assault" (extra combat phase) and mechanics like "exalted" or "training" that involve attacking aggressively, but none called "Relentless March."
If you can share the exact card text or where you saw this term, I'd be happy to explain how it works and its Commander implications. I'd rather give you accurate rules than invent an interpretation that could mislead you at the table.
Iconic cards with Relentless March
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