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God or Goddess in the story setting of The Rising of the Shield Hero are not "True Gods", but beings who have transcended and became an omnipotent, for all intents and purposes, existence. Although being called omnipotent, they are simply beings that have evolved to such a point where they are no longer bound to Laws of a World and have the ability to make small changes to Concept/law of a World with a set restriction. There are many Transcended Beings, or Gods, although all have reach the status of God, the method of becoming one can be different, which results in different abilities.
According to Ark, a god killer, so-called Gods are in fact incredibly ancient beings who come from equally ancient civilizations that have evolved into their present state of nigh omnipotence over time. For example as science and/or magic, whichever one the given society is running on, continues to evolve naturally a species will seek immortality, eventually they get it, then they continue to increase their power and find ways to leave their world and access others. Some races of this sort conquer other realities while others, usually for prideful reasons, choose to protect them, but regardless there always tends to be one amongst them who chooses to let all of that power go to his/her head, calls him/herself a god, and starts doing whatever they want with their power.
At some point in the past Ark, a god killer, emerged and began hunting gods, he also ironically aids responsible individuals in achieving godhood to aid him in this pursuit. There are many gods affiliated with him as allies according to Naofumi, but it is unknown if they predate him in origin or if they came to be because of him or if they simply learned of each other and allied over time.
Powers and abilities
Gods are nearly unstoppable for obvious reasons, the first god seen in the series, the main antagonist, Medea has two known forms of magic, Conception Magic, which basically allows her to kill a person just by imagining them dead, and Temporial magic, which allows her to attack a person in all divergent worlds and timelines where he/she exists. She easily killed Motoyasu, Ren and Naofumi, three of the Four Saint Heroes of Melromark, with this power and only spared Itsuki because she deemed it amusing to let what remained be destroyed by her army instead. She has also shown the ability to drag two worlds together, which she did with the eight worlds that comprise the Heroes World by the end of the story, this effect manifests as the Waves of Calamity. She did this because no single world can contain the sheer weight of her existence so she had to fuse eight together just so she could manifest herself there.
The only beings who appear to be able to defeat Gods are other Gods and Ark, who introduces himself as a god-killer and is ironically given the ability to one-hit-kill gods in exchange for being completely helpless against mortals. He is likely not a god himself, however Naofumi and Raphtalia are examples of gods who both lack one trait in exchange for another: Naofumi totally lacks the ability to attack in exchange for having near limitless defence and Raphtalia is the oposite, so it's not unlikely that Ark is a similar example, I.e. a god specialised in hunting other gods and nothing more.
The Heroes 0 Series weapons can hurt gods, as they were created using a potion that is designed to turn mortals into gods, however, the one time the heroes used this on Medea, all they managed to do was give her a cut on the wrist, which she promptly annihilated them for. The Heroes later recieve the undiluted potion and gain access to much stronger versions of these weapons which are much more effective, outright pummelling Medea during their second fight with her.
The main antagonist of the series overall. Medea is both the cause of the Waves of Calamity, and through them the murderer of everyone who died during said waves, and an omnipotent goddess possessing Conception and Temporial Magic. She is powerful enough to kill Motoyasu, Ren and Naofumi in quick succession while only getting a light cut on her wrist in return. She is overwhelmingly selfish and arrogant, a perfect example of a person with a God Complex who has enough power to back it up. She split off fragments of herself and reincarnated several dangerous people into the worlds she's fusing in order to have them undermine or kill the heroes in the worlds and prevent the Waves of Calamity from being stopped, one of these fragments is revealed to be Malty Melromarc, who turned the Hero of the Cane, Altcray, her own father, from a wise king into a fool, directly caused Naofumi, Motoyasu, Ren and Itsuki's downfalls one at a time, and even tried to use Tact to kill all of them. She shows no disregard or regret for her actions, as Malty she prompted Tact to kill her own mother, and later shameless praised said mother in the hopes that this would prevent her father from killing her, when she manifests as Medea, she goes even further by reviving the Queen as her puppet to lead her army and kill the heroes she once helped. Her final objective is to destroy the fused world and take all the remaining power for herself, essentially killing billions of living beings over countless years just to make herself a little bit stronger(by comparison to her already vast power). Nothing she says or does indicates she will not do so again or hasn't done so before.
a being of unfathomable age who has hunted and killed gods who cause trouble in the multi-verse for an age immemorial, he is either a god himself or something even more enigmatic than them, he implies in fact that he is part of an existing system rather than an individual acting alone. However Ark tolerates the existence of and even creates some gods, such as when he allowed Naofumi and Raphtalia to become gods to save the Heroes' world from Medea's rampage. while Naofumi could sense clear hostility and killing intent from Ark, it also showed that Ark had no intent to act upon it. He also tolerates dragons because they play a vital role in the world despite their inherently destructive nature. Ark is utterly helpless against mortals, a baby could kill him, but on the flipside gods are utterly helpless against him to the point where he openly admits that he could easily defeat Medea and save the Heroes world himself, but notes that in doing so he might end up destroying the world itself in the process due to it being a fused world and his powers being designed to combat foreign matter. He also notes to Naofumi and Raphtalia that they can ask him to end their lives if they get tired of being immortal, once again stating he could easily do so because they've become gods and are as such vulnerable to him.
To defeat Medea, after already being killed by her once, Naofumi spends an unfathomable amount of time wandering the dimensional void, crossing one world after another on his way back from his world to the Heroes World, and gradually learning how to borrow their power. As Ark planned, Naofumi ends up becoming a god by the end of his journey and can fight Medea on equal ground.
Like Naofumi Raphtalia winds up walking from Naofumi's world back to the Heroes world after their defeat at Medea's hands. Gradually she learns to harness the powers of the many worlds they pass along the way and, as Ark planned, she becomes a goddess capable of fighting Medea on even ground.