Rules FAQ:
- If you win by 20+ STR, the losing opponent must have at least 3 characters in order for you to be able to play this event. (Source)
House Frey.
Reaction: After you win a challenge in which you control an attacking House Frey character, if it was the third challenge you initiated this phase, choose and kill a character controlled by the losing opponent. (Choose and kill 3 characters instead if you won by 20 or more STR.)
Link: Decklists