The Master of Darkness is an Ambusher, a unit that can appear on the battlefield once the game has started.
What does he bring to your army?
His “Sharp Perception” ability makes him the ideal counterpart against the enemy's game of Scouts and Ambushers.
His “Shrouded in Darkness” ability allows him to get close to his targets without the enemy being able to attack him.
He can join some Hunters and turn them into Scouts.