When Mina attempts to buy a shield from the Ossex blacksmith, his brother surprises her with a duel. Winning the one-off boss fight unlocks the Hollower's Shield as a usable weapon option — opening up the entire parry / bash gameplay style.
FAQ
How do you start the Blacksmith Brother duel?
Attempt to buy a shield from the Ossex Blacksmith. His brother triggers the duel as soon as Mina tries to purchase. Winning grants the Hollower's Shield (IGN).