Here're the rules for the two promo cards, in a nutshell. Mix both cards into the deck, replacing one agent and one spy.
Merlin: After the spies have revealed themselves to each other, they put their heads down & put their thumbs up. Merlin looks up & sees who the spies are (while the spies don't know who Merlin is.)
Assassin: If the spies lose the game, before any of the team cards are revealed, the Assassin player guesses who Merlin is. If that player is correct, the spies win the game instead.
Note: These roles were included with other Avalon roles in The Resistance: Hidden Agenda