WARNING: The following contains spoilers for Demon Slayer Season 2, Episode 18, "No Matter How Many Lives," now streaming on Crunchyroll, Funimation and Hulu.
Demon Slayer's main antagonist, Muzan Kibutsuji, is the wicked Demon King, known for being the first demon to walk the earth and having the distinctive ability to create and control other demons from his own blood. A true villain whose sheer power and intimidation strikes fear into even the 12 Kizuki -- the strongest of all demons -- if Muzan achieves his goal of obtaining true immortality, he will be unstoppable.
However, one of his demons has proved to be different from the rest, possessing the unique capabilities to interfere with said goal. Nezuko Kamado is an anomaly among the demon race, being one of the few to have never devoured a human -- replenishing her energy by sleeping instead of drinking human blood -- and to retain some of her human qualities, including her protective and caring nature toward her family. But the "Entertainment District" arc's finale has shown another special ability, solidifying the fact that Nezuko poses the greatest threat to Muzan's existence.
As demons continuously consume humans in Demon Slayer, they grow stronger and evolve. Once they gain enough strength they develop their very own Blood Demon Art. Such power manipulates the user's blood to manifest a supernatural technique, which varies greatly in power and versatility and is unique to each demon. But like demons themselves, Blood Demon Arts are not invincible and are susceptible to Nichirin swords and sunlight.
Nezuko's Blood Demon Art, called Exploding Blood, allows her to ignite her blood into pink flames or even conjure it from thin air. Since Nezuko doesn't eat humans, she grows stronger by sleeping, taking a different route to unlock her supernatural ability in Season 1 during the battle with Rui, Lower Moon 5 of the 12 Kizuki. Her flames are only harmful to demons, increasing the power of her attacks and hindering her opponents' regeneration abilities, while leaving humans and objects unharmed.
Additionally, Demon Slayer's Season 2 finale revealed that her Blood Demon Art can also burn out demonic poison as she saves the lives of her brother Tanjiro, Inosuke Hashibira and Tengen Uzui from Gyutaro, half of Upper Moon 6's, Poison Generation ability.
This newfound skill is a huge game changer for Demon Slayer because it raises the question: what else can Nezuko's Blood Demon Art do? Though she is still a young demon, she has exhibited a strength and power that equate to the Upper Moon demons with a regenerative ability that surpassed Daki's, the other half of Upper Moon 6.
Perhaps Nezuko can burn out the demon blood in someone who's recently been turned into a demon. Does this new power raise her chances of overcoming Muzan's attacks, or even reversing his many heinous crimes? With plenty of room left to grow, her pyrokinetic Blood Demon Art will only become stronger over time, raising the possibility of further undoing demonic attacks on a human body, thus posing the largest threat to Muzan and his influence over the human and demon worlds.
What exactly makes Nezuko and her Blood Demon Art so special? The Kamado family has a connection to Sun Breathing, which is the original and most powerful of Demon Slayer's breathing techniques. It's seen in Tanjiro as he's able to perform the Hinokami Kagura, a ritual dance passed through the Kamado family that allows him to use Sun Breathing. While Nezuko can't use this breathing style, her ancestry may have influenced her fire-themed Blood Demon Art -- and may explain why she's able to differentiate herself in an extraordinary way from other demons.