You could play this card while your threat is over 20, but then you would just be Swift. Or, you could play this card while all your heroes are ready, but then you would just be Silent. Also, this card sure takes a long time to read for something called Swift and Silent. You should probably Just use Cram instead.
Swift and Silent
Event. Cost: 1.
Action: Ready a hero you control. Then, if your threat is 20 or less and this is the first time you played a copy of Swift and Silent this round, return this card to your hand instead of discarding it.
A marching host of Elves had come up silently... The Fellowship of the Ring
The Dunland Trap #3. Leadership.