The Demon King, also known as Muzan Kibutsuji, is the originator and supreme ruler of all demons in the Demon Slayer manga universe. He was the first person to ever change into a demon, and for more than a thousand years, he has been rapidly assembling his army, terrifying humanity and inspiring the development of the Demon Slayer Corps.
He is renowned across the entire globe as the most powerful demon of all time, an implacable adversary, and an unassailable monarch. His reputation, though, might only be accurate because of one thing. All of Muzan's demons are under his control, but the few who have managed to escape his control possess enormous potential to develop into more powerful and advanced demons.
Demons Who Escape Muzan Gain Strength and Endurance
Muzan has full control over the will of the demons in his command. As such, his army never acts of its own accord and is micromanaged to the highest extent. However, there are some cases where demons are free of his control, and evolve into enhanced beings once they're untethered. The first example of this is Nezuko Kamado. After being turned, she almost instantly surpassed Muzan's influence and enhanced her regenerating abilities and speed. Within moments, she fought off the bloodlust and advanced her biology to regain strength through sleep instead of blood consumption. On top of this, it took Nezuko very little time to develop a unique Blood Demon Art that is harmful only to demons, all without the experience and human consumption the technique typically requires.
While Nezuko was the first example shown of demon evolution, she wasn't the first to change. Tamayo, the skilled doctor working toward a cure for Nezuko, is unrecognizable as a demon. She worked directly under Muzan as a servant at one point, but after seeing the legendary Yoriichi Tsugikuni come close to killing her master, hope returned to her. Tamayo's will to avenge her family and hatred for Muzan helped her break free of his control, and she evolved similarly to Nezuko. Consequently, she suppressed her instincts, ethically purchased blood instead of killing innocent people, and modified her demon functions with medicine so she could live a more human life. She advanced to an extent where Nezuko recognized Tamayo as a human, not a demon, resulting in the doctor feeling satisfied with her progress.
Nezuko and Tamayo both evolved to suppress their demonic biology so they could live among humans without being a threat, but one demon used this for a very different reason. Akaza overpowered Muzan when his human memories returned to him, reminding him of his will to protect his loved ones. The Upper Rank Three evolved into a demon who couldn't be killed when decapitated by a Nichirin sword, a unique ability no one else has in Demon Slayer.
Despite this, upon regaining human consciousness, Akaza recognized he was fighting for the wrong side and accepted defeat. He forcibly stopped his regeneration after being decapitated, but the Demon King's will returned to force him to live. By using his love for his family and respect for his past master, Akaza overpowered Muzan for a second time and passed into the afterlife with his loved ones.
Demons are Managed by Muzan Kibutsuji to Prevent their Evolution
Demons tend to evolve and unlock higher powers when they break free of Muzan's control, placing them above other standard demons. This means that if they kept their own will following their transformations, everyday demons would likely be far stronger than they are now. Enhanced agility, strength and endurance are quite basic evolutions, but being unkillable by decapitation is an advancement that poses a threat to humanity. If more demons could regain their will, there's nothing off the table in regard to their potential.
Thus, the reason Muzan has such strict control of his army could be to purposefully suppress them. There's no way he would be doing it to make the Demon Slayer Corp's lives easier, so there must be an ulterior motive at play. If Muzan is aware of a demon's capability to evolve, he is likely controlling them to ensure he is never dethroned by a stronger demon of his creation. This way, he maintains his reputation as the strongest and lives for thousands of years to come.
Demon Slayer's antagonist is exceptionally intelligent, and uses this to his advantage. By scaring demons into order he ensures their free will is lost, giving him complete control. Of the demons able to break free of his command, all have shown incredible evolution of their power, implying the Demon King holds back his subordinates' abilities. Muzan must maintain control over the demons to guarantee his place at the top of all demons and protect himself from mutiny.