Orochi/Synopsis

History
Orochi used to be a man who turned his back on humanity. Although it is unknown to what extent, he was also some sort of martial arts fighting genius.

Eventually, he crossed paths with Psykos, who swayed him to join her cause. Psykos turned him into a monster through a special "growth" stimulating process by putting him through many near-death experiences. Orochi became Psykos's greatest success after many failures and sacrifices occurring. As he began to transform from a human into a monster, his appearance began to change drastically, although he was not quite at the towering height he would eventually obtain. Psykos also conditioned his mind to be totally submissive to her.

Several years before the events of the story, Orochi battled and defeated Gouketsu. Instead of killing the martial artist, Orochi took him to the Monster Association Headquarters and gave him an offer to help the martial artist gain more power in exchange for cooperation.

Super Fight Arc
Gyoro Gyoro is first seen speaking with Orochi about the monster raid's situation, though he stays relatively silent. After Tatsumaki kills the Hundred-Eyes Octopus, Gyoro Gyoro finds the situation troublesome, but Orochi sees it as no concern because they can make as many Mysterious Beings as they please. Through his robot, Bofoi questions Orochi and Gyoro Gyoro about their goals, and while they tell him destruction is a natural part of a monster's instincts, it is not their end goal.

Orochi later kills and eats Awakened Cockroach for his failure to kill Genos and was about to do the same to Do-S, but spared her on behalf of Gyoro Gyoro's request. He realizes Gouketsu wasn't present at the meeting and has Gyoro Gyoro try to find him, but to Gyoro Gyoro's shock and worry, it discovers Gouketsu's head was cleaved from his body by an unknown hero.

Monster Association Arc
Bofoi mentions how Orochi destroyed one of his surveillance drones by impaling its thick armor with one of his horns. Moreover, the drone was unable to initiate the self-destruct mechanism before it was destroyed. This causes Bofoi a great deal of fear, and later he chooses not to help the Hero Association in their eventual raid because of it.

After Garou is captured and Gyoro Gyoro introduces him to Orochi, the Monster King gives him one day to bring back the head of any hero in order to prove his loyalty and join the Monster Association.

Later, when Garou returns to fight the Monster Association, Garou finds himself in Gyoro Gyoro's throne room and the two have a brief battle before Gyoro Gyoro decides to let Orochi take care of him. Orochi reaches through the floor and tightly grasps Garou in his hand, and the two move to the massive colosseum-like throne room to accommodate Orochi's massive size. The two begin to fight, and as the fight goes on Orochi begins to unveil his true appearance, unraveling his body into endless horns, mouths, and dragons, and Garou begins to take heavy damage from the onslaught of claws and fire. As the fight rages on, many Mysterious Beings are killed in the crossfire. Despite Garou's ferocious efforts, he is no match for Orochi's inhuman powers and is eventually sent crashing into a wall by a heavy blow.

Gyoro tells Garou to give up, for Orochi surpasses him in every aspect. But Garou derisively laughs and says that while Orochi is strong, he relies solely on his strength and has no real strategic or cognitive abilities, believing that he is a mindless monster and stating that he isn't at all afraid of him. Garou takes a fighting stance, but his eyes widen when he sees Orochi takes the same stance. Gyoro reveals that Orochi is in fact a fighting genius as well, and he was able to learn the Flowing Water Rock Smashing Fist just by observing Garou's movements. Orochi boasts to Garou that since he did not feel fear before, he will now grant him true terror. Alas, Garou is undeterred, and he passionately jumps up to strike Orochi. Despite this, Garou is easily knocked back, and collides into the wall yet again, knocking him unconscious and marking Orochi as the victor.

After the initial clash between the heroes and the monsters, Orochi and several other Dragon-level monsters are called by Gyoro Gyoro for a meeting. Soon afterwards, a small group of weaker monsters who fled from the clash are crushed by Gyoro Gyoro into a cube of meat and fed to Orochi.

When Child Emperor and Saitama begin heading towards the surface with Waganma, some of Orochi's dragons attack them from behind. Saitama quickly disposes of them, so fast that Child Emperor didn't notice them, but Saitama makes a mental note that something bigger is waiting beneath.

Gyoro Gyoro calls for Orochi to save her after being utterly decimated by Tatsumaki, although Orochi is unable to hear her. This is because Orochi has already started climbing through the rubble of the Monster Association base to personally meet Saitama, increasing his size several times over and spewing streams of fire. Upon meeting him, Orochi realizes that Saitama is not trash that can be ignored and attacks him, only managing to singe the end of his cape. The nearby Overgrown Rover is awoken by the struggle after having been knocked unconscious by Saitama earlier. Orochi orders Rover to kill Saitama, but it flees with its tail between its legs when it catches a glimpse of Saitama's face.

Orochi is shocked by Rover's disobedience and asks Saitama just what he did to him. When Saitama replies that he went easy on him, Orochi realizes that he is the one who killed Gouketsu and Elder Centipede, two of the Monster Association's major executives. Orochi gets excited at the thought of such a powerful opponent and condenses himself into a slimmer, more humanoid form to fight at full force. As he properly introduces himself Saitama cuts him off, telling him his expectations are already low enough. Orochi then relentlessly attacks Saitama, his blows shaking the entire landmass of Z-City.

Several punches and enormous fiery blasts later, Saitama launches himself through Orochi's skull, having ripped the Monster King's body to shreds over the course of the battle. Orochi asks Saitama what happened and Saitama replies that it was just a normal punch. His fractured head then falls into the abyss, now knowing true terror for himself. Orochi's body is then discovered by one of Gyoro Gyoro's drones just before Gyoro Gyoro is defeated by Tatsumaki.

However, Orochi ultimately survives Saitama's punch. His body, splattered across the Monster Association base, slowly reforms, absorbing all monsters unfortunate enough to come near him. He senses that Gyoro Gyoro is gone and Psykos herself is fighting Tatsumaki. Orochi blasts at Tatsumaki from all directions with his scattered body parts, allowing Psykos to escape. He then shoots himself towards Psykos, engulfing her in his biomass and attempting to absorb her. Psykos is shocked at the power Orochi was hiding, while the Monster King mockingly thanks her for the countless monster sacrifices and prepares to consume her too. However, Psykos manages to overpower Orochi, and they combine into a human-monster hybrid form.

With this new form, Psykos is in command, but Orochi maintains a level of control, openly stating through Psykos that he would find Saitama and consume him. Orochi and Tatsumaki clash, and combined with Psykos, they manage to increase the gravity around Tatsumaki while simultaneously channeling her energy into her barrier, increasing the internal pressure and dealing serious damage to Tatsumaki.

While Orochi is fighting against Tatsumaki, a part of his mass elsewhere attacks Flashy Flash, Saitama, and Manako. The flesh grabs Manako in an attempt to absorb her, but Flashy Flash saves her. Orochi then grabs Saitama's head, but his lack of hair causes the tentacles to slip off. Saitama punches the mass of flesh in retaliation, killing it. Another part of Orochi attempts to attack King and Tareo, but a boulder falls into the bucket of water acting as a levy for the elevator, increasing the rate of ascent and causing Orochi to miss them.

Orochi's main body yells in pain after feeling Saitama's punch, releasing Tatsumaki from his grasp. Orochi notices the massive number of cells that just died, wondering what the slippery feeling he felt was. After Tatsumaki hears Tareo is confirmed to be safe with King, she reveals she was holding back, and prepares to fight Orochi and Psykos at full power.

After Tatsumaki inverts the Monster Association base above the ground, Psykos and Orochi emerge from the top with an explosion. The villainous fusion showcases its power with a devastating energy beam that cuts through the very Earth's surface. Psykos brags that she (and therefore Orochi) was somehow bestowed a power superior to Tatsumaki's, and is now a perfect lifeform. When called an idiot by Tatsumaki, as well as being called out for being forced to rely on Orochi for everything, Psykos flies into a rage, sending a barrage of dragons and energy blasts at her. Throughout most of the fight, Orochi is quiet and focused, letting Psykos do the talking, although there are still moments where he visibly threatens to take charge of the fusion, as shown by Psykos' face deteriorating to resemble his.

Tatsumaki pulls a fast one on Psykos, tricking her into attacking a rock dummy, which allows Tatsumaki to enter Psykos' giant body through Orochi's tendrils. Tatsumaki destroys the projection, but Psykos survives and begins reforming her body as Orochi impales both of Tatsumaki's hands and strikes her with psychic power. Psykos prepares to finish Tatsumaki off, but she is saved by Genos, who launches an energy beam of his own to intercept. Psykos and Orochi trade beams with Genos long enough for Tatsumaki to rescue the infiltration squad, allowing her to return to the fight and begin twisting the entire city in order to crush the roots of the creature. She then begins twisting the fusion itself, causing both Orochi and Psykos to roar in agony.

Desperate, Psykos and Orochi transform into a jet-like organism and move to consume the blood from the humans in the city. Tatsumaki senses that their presence is divided into seven due to Psykos using the same dummy technique she herself used before, so she creates a giant psychic barrier surrounding the area to halt their escape. Psykos attacks Tatsumaki with a laser, believing the hero is using all of her concentration to maintain the barrier so large, but she ends up hitting a dummy instead and accidentally revealing her own location.

Drive Knight then joins the fray to pursue Psykos and Orochi, engaging in a dogfight with the monstrous fusion. Genos also joins in, working together with Drive Knight to slow their movements and finally allowing Drive Knight to slice off a piece of Orochi, successfully capturing a fragment of him. This allows for the rest of the S-Class heroes to organize and get some hits in, damaging Orochi so much that Psykos flees his body. Now alone, Orochi continues to be pummeled by the heroes, leaving him a bloody mess on the ground, and Tatsumaki uproots the Monster Association ruins and turns it into a gigantic stone spear. She then drills the spear into Orochi, tearing him to shreds and pushing his remains into an underground ruin illuminated by lava.

The remain of Orochi is then used as a sacrifice, for Sage Centipede's emergence.