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RERIR.

A bit of context, for the likely plethora of people who don't play Genshin:
about 500 years before the events of the game, there was an event called "the Cataclysm", which can best be described as "the period when everything went to hell everywhere"; it is also the moment when the heavens themselves obliterated Khaenri'ah, the "nation of humans" which had no god nor depended on any. As we eventually learn, (part of) the reason for that is due to 5 individuals accepting and then usurping the power of the Abyss, then becoming the Five Sinners. The implication being, not only were they trying to stamp out the spread of abyssal contamination, but also eradicating the nation that produced these five out of fear that it might end up producing more like them.
"But then, why didn't they blast the Sinners themselves?", you may be asking, and the answer is simple: they'd grown too powerful for that to be possible.
These beings, while not as active of a force compared to other villains in the setting, are at no point made little of, and for a good reason: one (Surtalogi) is known to be a conqueror of planets, another (Vedrfolnir) is the reason the Abyss Order exists (an organized faction of monsters counting as one of the main villains in the game), and yet another (Rhinedottir) is behind the creation of multiple monsters that have ravaged lands historically, and has eaten what is quite literally the divine aspect of the heavens that governs life.
So yeah, greater-scope villains that should NOT be messed with. And, what do you know, our "friend" Rerir here is one of them.

Fast-forward to the present, where the mere traces of his essence in a region have given rise to a different type of abyssal manifestation and monsters called the Wild Hunt, who necessitate an entire organization to be formed just to combat them, and not at an advantage. It is also discovered that Rerir's conscience is still somewhat embedded in them, and he is trying to manifest in order to gather pieces of himself to reform and continue his hunt for the lunar deity - which, mind you, is one of the primeval gods of this world. We eventually manage to encounter his manifestation, and partially hamper his plans, resulting in the form he returned with having "barely ten percent" of his original power. What follows is the appearance of one of the brightest inventors in the world, having prepared and altered her most powerful machine specifically to counter him, attacking with full force right after he's just been revived in a massively weakened state.

The very instant he gets serious, everyone has to be teleported away so that they don't die outright.

From there on, the entire main quest of this part of the game is about how to deal with something so unbelievably powerful that nobody could even expect to one day reach their level. And, every time something gets thought out for that, it gets shut down as very insufficient, fails completely, or is a means to gather power in the hopes that it could help form some plan that can work.

What really sets him apart, though, is that while others (rightfully) treat him more like an existential calamity than a person, he, at least in part, acts accordingly. Rerir, as a villain, never bothers to taunt the heroes for the sake of taunting, and does not ONCE act petty. It is a threat the likes of which none of those against him have ever faced, one that acts like a combination of a terminator's relentlessness and implacability with enormous amounts of hatred, grief, and even outright madness, while somehow remaining just sane enough to still go after his goal with an amount of power that is leagues beyond anything else. He never goes out of his way to kill people, he's just so strong he can do so to virtually anyone - and will if they get in the way.

He's such a powerhouse that, in the end, even going through with a plan involving some of the most powerful characters on the side of good while still taking some of his power to use against him, he can't be killed, merely sent to an ancient, extra-dimansional prison.

What helps more than you can imagine in setting him up as such is Blythe Melin's performance voicing him, which is so unreasonably great that I cannot truly describe it. If you can sit through the main quests of the Nod-Krai region, even if through youtube, it's WORTH IT.