After the destruction of A-City, Iaian climbs out of the rubble only to see nothing standing in front of him but the Hero Association's Headquarters. He later saves a father and his son from being killed by Melzargard after the father moves some rubble to get to his son. As he is standing there, the monster appears behind him and attempts to crush his head. Melzargard only crushes his helmet, its occupant having moved away in time. He states that there was no way he would not have noticed its killing intent and proceeds to attack the monster. While dodging one of the monster's attacks, his left arm was torn off. Melzargard then began splitting itself into five different forms.  Feeling hopeless of his situation, the monster is suddenly cut to pieces by Atomic Samurai and Iaian was ordered to stop bleeding by his master. 
While the S-Class heroes are fighting the monster, Iaian advised them to retreat and find another way to kill the monster, but his pleas were futile. During the fight he witnessed Silver Fang being struck by Melzargard's attack and was surprised to see he was unharmed. After the monster was defeated, the spaceship that was above them began to collapse and Iaian was helped carried by Atomic Samurai.
Iaian, Okamaitachi and Bushidrill are waiting outside discussing about Garou's hero hunt, while their master Atomic Samurai is attending a meeting with the Council of Swordmasters. After Atomic Samurai killed the monsterized Haragiri, Iaian noticed the smell of blood and Atomic Samurai ordered they will strike the Monster Association.
Iaian is seen waiting for the S-Class meeting to end with his fellow disciples.
Iaian is one of the heroes who fights the first wave of monsters from the Monster Association. When Rhino Wrestler appears from the 2nd wave, Iaian watches the other heroes attempt to attack the monster but fail miserably.
The A-class hero unsheathes his sword and strikes his horn, momentarily halting the monster's movement.
Rhino Wrestler rates his attack 69 points, and the others see it as the opportune moment to strike together. First, Jet Nice Guy attacks with his Ultimate Jet Straight across the monster's face; next, Double Hole tickles his nose to activate his energy sneeze. Crescent Eyebroll uses his Crescent Consecutive Slashes, while Narcisstoic uses his Rose American Sack attack, and the rest of the heroes join in, but Rhino Wrestler is able to blow all of them away, rating each of their attacks. The heroes are shocked at his power, while Rhino Wrestler reminisces of a moment when Gyoro Gyoro conversed with Rhino Wrestler before the monster raid and how Gyoro Gyoro commended the monster for the increase in strength. Rhino Wrestler screams out for the strongest heroes to fight him, all-the-while fighting Iaian. Rhino Wrestler is able to hold Iaian back, but before the battle could continue further, Atomic Samurai tells his disciple he would take his place. Iaian insists he could handle it, but Atomic Samurai tells him he would cut off his horn himself.
Iaian, Bushidrill, Okamaitachi encounter Do-S and the mind-controlled mercenaries. They do battle but are interrupted by Sweet Mask. Sweet Mask calls for the swordsmen to leave the battle to him. They leave and encounter Pig God eating The Great Food Tub and decide to take another route.
The sword heroes encounter Devil Long Hair, and Okamaitachi becomes infatuated with him. Iaian and Bushidrill realize Okamaitachi is infatuated with the monster. Bushidrill and Iaian tell Okamaitachi to snap out of it, reminding them that their role is to lighten their master's burden. Okamaitachi attacks with his Air Blade, but Devil Long Hair stops it with his hair, shocking the swordsmen. Devil Long Hair counterattacks, but Bushidrill manages to stop the attack before any harm was done. The demon-level monster then extends his hair toward Iaian and Bushidrill, and the swordsmen prepare for battle.
The three swordsmen work as a unit, rushing in and cutting Devil Long Hair's hair as fast and as effectively as possible, and manage to defeat him, although they admit it was a difficult opponent, and had they not worked together, they surely would have lost. But Okamaitachi, Iaian, and Bushidrill are attacked by Evil Natural Water, a Dragon-level monster, who defeats all of them and badly injures them, knocking Okamaitachi and Bushidrill unconscious. Child Emperor comes across them, and Iaian, the only one who is still conscious, urges Child Emperor to leave, as the monster is still lurking around.
The three swordsmen are then swallowed up by Pig God, who found them and consumed them, and manages to bring all three of them up to the surface and regurgitate them without any noticeable injuries. As Atomic Samurai watches the other S-class heroes fighting and being unable to do anything since he lost his sword, Iaian speaks to his master and gives him his sword to fight Garou.
Appearances in Other Media
Iaian was invited along with the S-Class heroes to a hot spring resort, after the Dark Matter Thieves failed invasion. While enjoying himself at the hot spring, he wasn't satisfied that some of the heroes that did not participate in the battle were invited to the resort, but Bang managed to calm down the situation. The next day after the ceremony, the heroes find Zombieman murdered, stabbed in the back by Atomic Samurai's katana.
As Child Emperor interrogates every hero, Iaian was one of many alibis for Atomic Samurai so all suspicions were dropped. While the investigation wasn't progressing, Tatsumaki tried to restrain everyone, until Zombieman suddenly woke up. Zombieman later revealed that Atomic Samurai's katana was floating with green light surrounding it similar to Tatsumaki when it stabbed him. Zombieman didn't care about what happened the night prior and the heroes disbanded.
Bang asks Atomic Samurai how Iaian is dealing with the loss of his arm at the hands of Melzargard, and Atomic responds by saying that Iaian is strong, and has already started training to get adapted to fighing with just one arm.