It's called "Ai Shuang's Revenge". Xiao A's sword is called Frostmourne. According to the plot of the mission where DK was born, you were the one who died at the hands of the Scourge and was resurrected by Xiao A. You also killed your friend (i.e. "Sorrow") with your own hands. Later you found yourself, betrayed Xiao A, picked up the frozen sword (i.e. "Frost") in your hand, and also responded to Shadow's Sorrow ), joined the expeditionary force, entered the ranks of the Northern Expedition, marched into the Ice Crown Fortress with his friend ***, killed Xiao A, and avenged his friends and himself (ie "revenge"). Are you satisfied?