Shadow (1).
Reaction: After you win a challenge in which you control an attacking Raider character, look at the losing opponent's hand. Then, choose 1 card in that hand and place it on top of its owner's deck. (Choose up to 2 cards and place them in any order instead if you played this event from shadows.) (Max 1 per challenge.)
Link: Decklists
Rules FAQ
Up to 2 includes 0, so you can choose no cards if you play this event from shadows. But you have to choose a card if you play it from your hand.
If you have Late Summer Feast revealed, the forced reaction will trigger first. If the opponent declines to draw a card, they do not get another chance after this event is played.