During setup, you can optionally replace one Loyal Servant card with Percival. (If you are playing with only 5 players, you should also replace the Minion of Mordred with either Mordred or Morgana.)

The player who receives the Percival card starts the game with knowledge of which other player is Merlin. Alternatively, if Morgana is also used, Percival sees both Merlin and Morgana but doesn't know which is which.