Dun Banner Medic is a 5-strength Siege unit with the Medic ability. When you play it, you pick a non-hero unit from your discard pile and play it again at once, so you get two cards on the board for one card from your hand.
That makes it one of the most useful cards in the early Northern Realms deck, especially in rounds two and three when your discard pile has something worth bringing back.
The single copy is in the Northern Realms starter deck Geralt gets when he learns gwent in White Orchard. No shopping needed.
