The Real Reason Muzan Is So Powerful In Demon Slayer
As the main antagonist of "Demon Slayer," few characters command as much power as Muzan Kibutsuhi. Don't let his pallid, sickly appearance and resemblance to the King of Pop fool you, Muzan is as ruthless as deadly as they come. His demonic nature compels him to kill and consume humans, usually because they've crossed him in even the slightest way. On the slight chance he doesn't want to kill you, he'll probably turn you into a demon like him. After over a thousand years of doing this, he has transformed and eaten an army's worth of people.
If, for instance, a member of the Demon Slayer Corps wanted to hunt Muzan down and slay him, then they probably wouldn't even make it past one of his cronies. Muzan is the father of all demonkind, and can make his subjects as powerful as necessary according to their abilities and his desires. But numbers alone isn't what makes Muzan strong. This mock Michael Jackson is strong enough on his own. The real reason Muzan is so powerful is due to a little blue flower and a bit of twisted cellular biology.
Muzan was human until he found the Blue Spider Lily
Muzan may be the first demon, but he's still only 1,000 years old (via Kimetsu no Yaiba Wiki). That's because in "Demon Slayer," demons aren't a product of deific origin, but scientific origin. Back when Muzan was a mortal man, he suffered from an incurable disease set to kill him before he reached the age of 20. Desperate, Muzan allowed his doctor to administer an experimental medicine derived from a rare flower known as the Blue Spider Lily.
At first, the medicine didn't seem to work. Enraged at the doctor's lack of progress in curing him, Muzan murdered the medical expert. Soon after, however, he learned that the medicine had actually transformed him. Stronger, faster, and virtually immortal, Muzan's new mutation only came with two caveats. The first is that he must eat humans to survive, which Muzan considers more of a minor annoyance than anything else. The second, and the one that Muzan really doesn't like, is that he disintegrates in sunlight.
Unfortunately, Muzan had just murdered the only guy who knows anything about Blue Spider Lilies. Thus, history's first demon set out to spend the next 1,000 years searching for the flower, to no avail. Even so, his demonic powers have allowed him to survive longer than any regular man could. And they've done so by altering the way in which his cells behave on a fundamental level.
Muzan's cells are the source of his powers
Upon consuming the Blue Spider Lily medicine, Muzan's cells gained numerous new abilities. The primary one, which grants him his immortality and enhanced healing, is limitless and rapid cellular regeneration. On top of that, Muzan has complete control over their function. Similarly to vampires and Pillar Men from "JoJo's Bizarre Adventure," Muzan is a biological freak of nature.
With his powers, Muzan can shapeshift into any form he desires. He is a living weapon, and his intelligence and resilience allow him to use his powers creatively. He often turns his blood into sharp brambles, sprouts new limbs, and modifies old ones to suit his needs. He can even absorb and consume humans just by touching them.
His cellular manipulation powers are also responsible for his ability to create new demons. When Muzan creates a demon, he does so by giving them a certain amount of his own blood. If they're strong enough to withstand the amount given, they survive and become a demon. This process also allows Muzan to establish a telepathic communication with them, as a small part of his consciousness lives inside them. Even if Muzan's main body gets killed, every demon carries a small part of him within them. Considering this, it's no surprise that he's the final boss of the series. Muzan's abilities outclass even the most skilled demon slayers — and if he ever finds the Blue Spider Lily, he'll become unstoppable.