Rules FAQ
Events cannot enter play, so if Ser Mark reveals an event it remains on top of your deck and he remains in play.
Knight.
Reaction: After you win a challenge in which Ser Mark Mullendore is participating, reveal the top card of your deck. You may put that card into play. If you do, place Ser Mark Mullendore on top of your deck (cannot be saved).
Link: Decklists
Rules FAQ
Events cannot enter play, so if Ser Mark reveals an event it remains on top of your deck and he remains in play.