Kethis's last ability affects only legendary cards that are in your graveyard at the time it resolves. Legendary cards put into your graveyard later in the turn won't gain the ability. If you play a legendary card and it returns to your graveyard, it won't have the ability anymore.
Spero che questo chiarisca tutti i dubbi che ancora ci possono essere. Se avete problemi credo che MaRo sarà contento di ascoltarvi