Spoilers for One-Punch Man, Japanese chapter 165 ahead!
There have been many major antagonists to appear throughout One-Punch Man's Monster Association story arc, from the Hero Hunter Garou to the Monster King to an evil God who seeks the extermination of humanity. One of the most destructive villains, however, was already defeated—or so it appeared until the latest chapter, when it was revealed that Psykos was still alive.
Despite being a human Esper with psychic powers like Tatsumaki (Terrible Tornado) and Fubuki (Hellish Blizzard), Psykos was revealed as the mastermind behind the Monster Association, using a fake monster body by the name of Gyoro Gyoro to control and influence them. After being exposed, Psykos and the Monster King, Orochi, began a bizarre fusion, leading to the creation of a kaiju-sized monster with incredible power that Tatsumaki was only just barely able to defeat by shredding the very Earth and crashing it down on Psykos-Orochi like an asteroid from space.
In chapter 165 (by Japanese numbering), in response to the appearance of a divinely-empowered Garou, Psykos reappears for the first time in 25 chapters, emerging from the rubble that Tatsumaki had left her buried in. Orochi was previously destroyed by Saitama, so there's at least no risk of the massive fusion coming back, but Psykos' survival at all may come as a surprise since she shouldn't have too much more durability than the average human. Tatsumaki's attack delivered a city-leveling amount of force, so presumably Psykos' own powers must have offered some degree of protection. What's interesting about her survival, however, isn't so much her potential to continue rampaging as much as what knowledge she may possess about this God, as she comments on this immediately upon seeing Garou's new look and abilities.
When Psykos and Orochi were fused, this same God-being offered them power, an offer that she was all too happy to accept, both to prove her psychic superiority over the sisters and to have that feeling of a divine cosmic destiny. This didn't help her to defeat Tatsumaki, however, so there's no reason to think that Garou is unstoppable. On the other hand, Psykos knows full well what kind of power this God being actually wields and that what she tapped into was only an infinitesimal fraction; the S-class heroes still have no idea what's going on, and the only one with any real awareness of this being at all is the often-absent top hero Blast. Whether out of a legitimate concern for the Earth (which has often been a rallying cry for One-Punch's monsters) or simply out of spite for being forsaken, Psykos could provide the heroes with some critical intel to, if not win, at least survive matching up with Garou in this state. Of course, there's also the possibility that if Psykos decides to aid the heroes, she may well be wiped out in exactly the same way God's other chosen, Homeless Emperor, was. Whether she wants to help or not, Psykos' fate may already be sealed.
While Garou may be the heroes' main concern for the moment, Psykos is currently a major wild card who could have some truly unpredictable impacts on how all this plays out. After all, even if Garou is defeated in this divinely empowered state, there's still the source of those powers to contend with, and as it stands now, One-Punch Man's heroes just aren't equipped to defeat a God.