With Muzan's defeat, all demons created through his blood died, leaving Yushiro and Chachamaru as the only demons that survived. With no demon bodies to preserve as well as recording historical documents of demons to be made, the knowledge of the demon race eventually fades into obscurity and mythology.
In the end, Muzan dies due to sunlight, and beheading, but turns his consciousness in Tanjiro, with the ability to walk in the sun. Demon King Tanjiro is equal to Demon King Muzan, but he can walk in the sun. Everyone can't bring themselves to kill Tanjiro.
The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo. Susamaru accidentally shot Muzan's name aloud, attempting to defend his honor with her mind clouded.
Picking up three months after Muzan's defeat, the penultimate chapter shows Tanjiro has recovered from his near-transformation into a demon. However, the injuries he sustained have lasting effects now that he is again human, with his left arm and right eye permanently damaged.
The explosion, which was enhanced by spike traps, manages to injure Muzan. Before he is able to regenerate fully, Tamayo appears and uses her Blood Demon Art to keep him in place, while using a drug that can turn him into human. Using this opportunity, Gyomei Himejima appears and destroys Muzan's head.
The Water Hashira, Giyu, survived the "Infinity Castle" arc after fighting at Tanjiro's side against both Akaza and Muzan.
Even 400 years later, Muzan's fear of Yoriichi remained fully intact, as simply laying eyes on Tanjiro's hanafuda earrings caused him to recall their clash.
Kokushibo
Kokushibo was the strongest swordsman and considered as the strongest member of the Twelve Demon Moons and the second strongest demon after Muzan Kibitsuji. As a hundred years old demon, he fought against numerous demon slayer and acquired tons of knowledge and experience about a battle.
Yoriichi Tsugikuni was the first Demon Slayer and also the one who invented Breathing Techniques in the first place. Yoriichi was born with the Demon Slayer mark and prowess in swordsmanship. His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape.
During his final struggle, and as a way to slow down the effects of the sun, Muzan adopts an enlarged demonic baby form, approximately as tall as a 3 story building when standing up. In this form he has very pronounced blood vessels across his body, small claws in his hands and fanged teeth in his mouth.
Fun fact: The reason why Enmu was able to say Muzan's name out loud without dying is because they're married!
Muzan spared Nezuko most likely because she expects something from turning a member of the Kamado family. True enough, Nezuko grew and transformed over the series. She demonstrated her Blood Demon Art during the fight with Rui. Nezuko then did her Demon Transformation in the fight against Daki.
Muzan's own blood is also shown to have special properties that can strengthen a demon, proportional to the amount of blood infused.
To sum things up, Tanjiro died fighting Muzan and was revived within seconds due to the transfer of blood from the Demon King to Tanjiro. He turns into a Demon and Kanao injects a certain medicine made from Wisteria flowers that turns him back to a human.
The Water Hashira, Giyu, survived the "Infinity Castle" arc after fighting beside Tanjiro against Akaza and Muzan.
Being a demon, Muzan is capable of living for many, many years. However, unlike other demons that have aged over hundreds of years. Muzan is well over 1000 years old. Despite achieving an age that no one else has, his appearance makes him look like an adult man in his early to mid-30s.
However, Tanjiro is not a descendant of Yoriichi Tsukiguni. Tanjiro is the descendant of someone who knew Yoriichi personally and was a good friend of his as well. Sumiyoshi was rescued by Yoriichi when a demon threatened his life. Since then Yoriichi often met him and they became close acquaintances.
11. The demon slayers successfully defeated Muzan. Now the world can finally find peace. However, before they can celebrate their victory, they realize that Tanjirou has died when preventing Muzan from running away.
Tanjiro decapitates Akaza with the Hinokami Kagura: Setting Sun Transformation. Akaza was bewildered, wondering how Tanjiro did not have any fighting spirit, comparing this feeling to fighting against a plant. He acknowledges that Tanjiro surpassed his speed.
As a result, Tengen Uzui has to deal with the moniker of being the least powerful Hashira around. Of course, that's not the worst thing in the world, as all Hashira are strong. He's still incredibly powerful and can give any demon a run for their money.
When it comes to pure strength, Shinobu Kocho is the weakest Hashira in Demon Slayer. She doesn't even have enough strength to decapitate a demon's head and she makes up for it by injecting poison into the demons.
1) Demon Tanjiro
It is important to note that Tanjiro also received every last drop of Muzan's blood, which was way more than what any other demon from the Upper Moon had. As a result, he became the strongest demon in Demon Slayer.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
Yoriichi was incredibly proficient in his Sun Breathing, being able to take down demons effortlessly and even overwhelm the Demon King and strongest Upper Rank.
The only thing he's weak to is the sun. That is why he turned other humans into demons and spread his blood so that one of them might find a solution against the sunlight, so he can become truly perfect.