Fateful Showdown deals damage to any target equal to the number of cards in your hand. Discard all the cards in your hand, then draw that many cards.
9/20/2016 The number of cards in your hand is counted only as Fateful Showdown resolves. Fateful Showdown is on the stack at this time, so it’s not counted.
9/20/2016 If Fateful Showdown’s target becomes an illegal target, you won’t discard your hand or draw any cards.
1/19/2018 There are many important moments in the story, but the most crucial—called “story spotlights”—are shown on cards. These cards have the Planeswalker symbol in their text box; this symbol has no effect on gameplay. You can read more about these events in the official Magic fiction at http://www.mtgstory.com.