"Fallen Warrior" isn't an official Magic: The Gathering keyword or ability—it doesn't appear in the comprehensive rules or on any printed card as a game term. You may be thinking of something else: there's the creature type "Warrior" (a tribal type that matters for cards like Chief of the Edge or Herald's Horn), or perhaps a specific card name, token, or flavor reference you've encountered.
If you meant a mechanic like graveyard recursion (bringing creatures back from the dead), relevant keywords include Unearth, Embalm, Eternalize, Disturb, or Encore—each with its own rules nuances about exile, tokens, and timing.
Could you clarify the exact card name or context? If it's from a custom set or unofficial format like one hosted on Mythicwyrm, share the card text and I'll happily break down how it works in Commander.