WARNING: The following contains spoilers for One-Punch Man's "Monster Association Arc," available in English via Viz Media.
Despite being a parody, One-Punch Man has some of the grandest battles in the world of animanga -- and Yusuke Murata's skillful depictions heighten the tension of each one. The series' longest story arc, "Monster Association," also has the best fight scenes.
Tis particular arc is all about the big clash between the Hero Association and the Monster Association, with both sides sending out their strongest fighters. Unsurprisingly, there are quite a few casualties during such an event. While the Monster Association is almost exterminated with most of its executives gone, the heroes have also suffered tragic fates. Here are the worst among them.
Blackluster Lost His Luster
Like most heroes in One-Punch Man, Superalloy Blackluster wasn't strong from the start. He used to be meek and couldn't win at anything, only gaining his lustrous muscles and immense durability after years of weight training. Blackluster is strong enough to obliterate a monster with a mere clap and durable enough to withstand lethal high-pressured water attacks from Evil Mineral Water, taking it as nothing more than a body scrub. However, he isn't invincible.
In the squabble against Barf President Ugly and Black Spermatozoon, Barf President Ugly's acid -- which was shown to be potent enough to easily melt a hero -- got into Blackluster's skin and immediately started to melt his flesh. The realization of his hard-earned muscles melting away shook his mentality. To make matters worse, Black Spermatozoon fused trillions of his little selves and became Golden Spermatozoon. The monster's shiny and brilliant muscles were the final blow to Blackluster's self-confidence, completely rendering the hero unfit for battle and leaving him with shattered pride.
Tank-top Master's Heart Stopped Beating
One-Punch Man's Tank-top Master is an S-Class hero and the leader of the Tank Topper Army, a group of heroes who believe in the supremacy of tank tops. He's a chivalrous man who doesn't care about admitting his limitations, as proved when he admitted being weaker than Tornado. Tank-top Master has immense strength and can easily throw gigantic chunks of concrete like a ball.
Unfortunately, the tank top believer was sent to the hospital prior to the Monster Association invasion. He missed most of the action but was able to rejoin the fray at a critical moment. In Chapter 141 he assisted Pig God in rescuing the members of the Narinki Private Force, hurling his partner after the eating hero put the unconscious soldiers in his mouth. However, that made Tank-top Master a target of President Ugly.
Tank-top Master met the villain's strike with an attack of his own, but his strength fell short, resulting in both of his arms being mangled. President Ugly followed up with consecutive Full Body Destructin' Punch, leaving the poor hero in a crushed state. He managed to escape death when Genos pulled his tank top and lost grip. The piece of clothing snapped back, restarting the hero's heart and proving once and for all that the tank top really is indestructible.
Atomic Samurai Watched Other Master Swordsmen Die
Atomic Samurai is a noble swordsman who acknowledges the skill of other master swordsmen and regards them with high respect. He invited the entire Holy Order of the Sword during the battle against the Monster Association, and they heed his call. However, their appearance was cut short by Barf President Ugly.
Zanbai was the first to taste the villain's digestive ability after being hit in the face by his fluids, immediately melting the part that was touched. When Barf President Ugly executed a sneak attack, he also got to kill Amahare and Nichirin. The latter bestowed one of the strongest swords in One-Punch Man -- the legendary Nichirin sword -- to Atomic Samurai with his last breath and told him he was the only member of the Holy Order of the Sword.
Most S-Class heroes in One-Punch Man are arrogant since they have the power to back it up, but that doesn't mean they won't ever fall from grace. The events during the Hero Association and Monster Association fight are proof of that.