Avallac'h is an exceedingly powerful character in the main game, but is equally effective on the tabletop. He is a spy card that gives your opponent added attack points, but allows you to draw 2 cards in exchange.
As a spy, he is basically perfect, because he does not give your opponent any extra attack. He is also a hero card, which means he cannot be stolen and used against you. Lastly, he is neutral and available to any faction. This is especially useful for Scoia'tael and Monster decks, which have no spies of their own -- compared to Nilfgaard and the Northern Realms, which have 3 each.
Avallac'h won't be available to you until you reach the Skellige Isles. It is won by defeating Gremist in Gedyneith, on Ard Skellig. By this point, you will have leveled up sufficiently and your card collection will be extensive, so you will have no issues obtaining this card.
Utilizing spies is one of the strongest tactics in the game. For more information on how to do this, follow this link and scroll down to the spy section.