After Genji agreed to the man's (akil) proposal,he made an agreement with the man that he would take him to meet Mikasa and also bring his father back from death,akil agreed to the deal and told Genji that he would destroy the barrier tonight, Genji agreed and went back to prepare for the operation.
When Genji went back home he saw everyone gather in the kitchen and suddenly remembered what he did to his cousins but he simply walked right past the kitchen behaving as if nothing had happened, Genji went back to his room and wore his fighting uniform and went to his closet to carry his sword,the sword that was made by his father the god of thunder,an unbreakable sword capable of slicing anything,it was the sword he used to kill the spirit king Sylvia who took the power's of the 4 great spirits and wanted to destroy the demons and humans, Genji wielding the black lightning sword slaughtered Sylvia and his entire army in seconds,it is because of that sword he was given the name "BLACK SAMURAI" after preparing his artillery Genji went out to fulfill his part of the bargain,as Genji stood on the roof of the castle and set out as he whispered the words "DEATH" on that faithful night one by one Genji slaughtered every soldier that was responsible for creating the barriers,he attacked them so fast they couldn't attack back, they couldn't even see his blade, he slaughtered every single soldier leaving without a trace he was ever there.
Osiris the commander of the emerald city's forces was gazing at the sky until he could no longer feel the presence of the barrier ,he immediately teleported to the tower where the soldiers who were creating and maintaining the wall only to see their lifeless bodies and blood splattered all over the tower, Osiris immediately teleported to the castle and alerted the otsuki of the impending danger.
Genji reached his room and met Hamak there, Hamak there, Hamak asks Genji where he went and Genji wept bitterly and told Hamak what he had done and also said that Hamak has to die, Hamak replied saying "I don't want to fight you Genji"and Genji said "this fight will end in blood and tears"Genji also pleads that Hamak forgive him for what was about to happen would be the end of their friendship but not the end of their brotherhood, Hamak says "then let the fight begin". ONE SHALL DIE AND THE OTHER SHALL LIVE.