Dugu Qiubai, literally "Loner Who Seeks Defeat", is a fictional character who is mentioned by name in three wuxia novels by Jin Yong (Louis Cha). He does not appear in any of the novels because he lived in an era long before the events of the novels took place. Nicknamed "Sword Devil" (劍魔) to reflect his prowess in and devotion to the practice of swordplay, he attains the philosophical level of "swordsmanship without a sword", which means that he has reached a level where he is better without sword. This means relying on his own limbs and inner energies far exceeds any tools created by humanity.
In his resting place, Dugu Qiubai had divided his martial arts life to five stages and is the creator of the legendary Dugu Nine Sword Skills.
Before the age of 20, he used a very powerful steel sword. He states, unyielding, sharp and ferocious. He had dominated an area of China that was known for having many powerful warriors. The unyielding, sharp and ferociousness also signifies a certain part of his life being young, headstrong and stubborn which are common traits of teens and teenagers which is most likely to be made worse when no one can kick this teen's butt.
Before the age of 30, he used Ziwei Soft Sword, nothing is canonically known about this sword other than speculations as Dugu Qiubai had discarded it into a chasm stating that it was cursed and that he accidentally hurt a good person with it. Speculations: People generally believe that Dugu Qiubai used the flexibility of the sword to his advantage which also signify a certain stage of his life where he learns to be more flexible and less hotheaded.
Before age of 40, he used Darksteel Heavy Sword. A heavy sword made out of materials found in meteorites. (Suspected to be Tungsten and Titanium) It has no sharp edges and is more cumbersome. Yet Dugu Qiubai was able to dominate foes who generally use high speed and complex techniques which shows Dugu Qiubai's high intelligence and ability to think out of the box. Yang Guo was led by Dugu Qiubai's pet Condor to trained with this sword after Guo Fu accidentally cuts his arm off while suffering from Passion Flower's poison. Yang Guo stated that after losing an arm and barely surviving massive loss of blood, he would only be average even after he recovers. However, through Dugu Qiubai's incomplete training methods copied by the Condor, Yang Guo was able to elevate himself above strongest warriors such as Guo Jing and Jinlun Guoshi/Fa Wang despite being handicap in just under 3 months. This is even more impressive considering Guo Jing and Jinlun Guoshi were both far above his level before he got his arm chopped off. It is to be noted that despite having incomplete training, Yang Guo was unbeatable while wielding the Darksteel Heavy Sword.
After age of 40, Dugu Qiubai used a wooden sword (sometimes a simple twig) to represent this stage. He states that after the age of 40, he is no longer bound by physical objects. By using a wooden sword, he was already stronger than Heavy Sword when he was in his 30s. He states that at this stage, he was approaching the stage where no sword beats any and every sword.
People speculates that since Dugu Qiubai was someone born in Xixia, which one of the 3 protagonists Demi-Gods and Semi-Devils finally settles in. It is likely that Dugu Qiubai might've been Xu Zu's son or grandson. Xu Zu being the leader of Lingjiu Palace, overall Master of the Xiaoyao Sect and Husband of the Royal Princess, being a noble would make sense for a teen to have the money and manpower to forge a powerful sword. In addition, Dugu Qiubai was unstoppable since he was a teen, this means he must've come into contact with powerful martial arts from young. Combined with very high innate talents and intelligence, it becomes believable that he would be far stronger than anyone else.
Being descendant of Xu Zu also added a likelihood that he might've inherit a perks of the several hundred years of extremely high quality Inner energies that Xu Zu received during his adventures in Demi-Gods and Semi-Devils. However, this is all a speculation.