A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Return of Myth Each Nonhuman Personality gains +1F/+1C. Each Nonhuman Follower gain +1F.
Will this card last until the end of turn or until the end of game? If it's the latter, will it only affect the nonhumans already in play or will it also affect the nonhumans that came into play afterwards?
Return of Myth Each Nonhuman Personality gains +1F/+1C. Each Nonhuman Follower gain +1F.
Will this card last until the end of turn or until the end of game? If it's the latter, will it only affect the nonhumans already in play or will it also affect the nonhumans that came into play afterwards?
It's a one-time effect, only affecting cards currently in play and lasting until the end of the turn (by default, due to there being no stated duration).
I'm sorry, I can't recall or find documentation of how that Event may have been ruled to work when it was tournament-legal. However, this ruling on a card with nearly identical wording supports my interpretation.