The Ominous Future

I'm the Ominous Future. Disaster level "God". The Ominous Future (な, Fukitsuna Mirai) is the bad future that has happened during the fight between Saitama and Garou. After defeating Garou, Saitama changes this future by reversing time.

Event
Having been mutated by "God" and achieved his Cosmic Fear Mode that results in incredible power, Garou battles Saitama once again. After hitting Saitama with his Gamma Ray Burst and sending him flying away, Garou confronts many heroes on the surface of Z-City in the absence of Saitama. Due to being influenced by "God", the cosmic radiation emitted from Garou's body quickly causes the heroes present to faint and threatens all life on Earth. Seeing such a massive threat, Blast, the S-Class Rank 1 hero, shows up and attempts to stop the Hero Hunter. After a brief scuffle between the two, Garou decides that Blast isn't worthy of his time and switches his attention to Genos and impales the cyborg through his chest before tearing off his power core, killing him. Later, Saitama returns and is depressed to discover that his disciple and many other heroes have been killed by Garou, while Blast watches Saitama face Garou and warns him to stay back. Enraged, Saitama charges at Garou with a Serious Punch, which Garou copies and attempts to counter-act with his own Serious Punch. Blast senses the immense power contained in their fists and immediately teleports them to space out of fear that the planet will be destroyed. Now outside of Earth's atmosphere, Saitama and Garou collide their fists which creates a Serious Punch² so powerful that the energy released from it is threatening to destroy the Earth beneath. Seeing as Blast is unable to handle the energy on his own, three of his comrades appear and offer him help and together, they manage to redirect the energy of the attack away from the Earth. Saitama and Garou are subsequently launched to Io, the moon of Jupiter, by the recoil of their own punches. The two of them resume their battle, with Saitama ultimately emerging victorious and Garou regretting his actions that led to Tareo's death. Repentant of this, Garou decides to pass on to Saitama his most powerful technique that involves the manipulation of time in hope that Saitama can master it and use it to travel backwards in time. Moments after executing his technique, Garou is killed by "God" but not before Saitama manages to get hold of the technique and successfully begins to travel through time. Saitama goes back to the past, bringing with him the core of the deceased Genos from the future, and arrives at the moment when Garou appeared in front of many heroes. With a single punch to the face, he utterly defeats Garou this time without letting the Ominous Future happen again. Now having successfully changed the future, Saitama is forcibly thrown back into his "past" self and becomes one with him, unable to remember what's happened. Suffered defeat from a mysterious knockout punch, a confused Garou screams in pain as God's powers begin to leave his body, while the heroes present are shocked to watch him.

Aftermath
After returning to the past and becoming one with his "past" self, Saitama is now unable to remember what's happened but the core of the deceased Genos from the Ominous Future is still in his hand. Saitama is shocked to see the depleted core and worried about the disciple while the now-alive Genos rushes towards him. Seeing that the core in Saitama's hand resembles his own core, Genos decides to check on it by plugging a sub-connector into it in order to make it function. Instantly after doing so, the memories contained in the core that had crossed time and space with Saitama flow into the cyborg and momentarily stun him.

Genos soon asserts that the core is indeed his and wonders if those memories from it are from a bad future that could have happened. He then tries to retell everything that happened in the Ominous Future to Saitama in a very lengthy way, which confuses Saitama who doesn't want to listen and simply tells him to stop. Believing that Saitama is pretending not to remember and that he is the only one else who knows about the fact that Saitama had crossed time and space to save everyone, Genos smiles and tells Saitama that he is a true hero, while thinking that he must let others who need to know be aware of this as well.

Trivia

 * In the original webcomic, after his fight with Superalloy Darkshine, Garou claimed himself to be the Ominous Future when confronting Fubuki. Unlike in the manga, what Garou had done after his proclamation was not that disastrous, as no humans or heroes had died from his actions.