The one Jin Yong martial arts that has probably spurred more arguments and debate than any other is probably Dugu 9 Jian. It is supposedly the legendary creation of Dugu Qiubai. We only see it in action in Xiao Ao Jiang Hu, as used by Linghu Chong and Feng Qingyang. The only other glimpse we catch of Dugu Qiubai is in Shendiao Xialu where Yang Guo encounters Dugu's grave, and the swords left behind describing his martial arts progression.
A very common misconception that I often see voiced here is that Dugu 9 Jian is somehow one of the EARLIER forms of Dugu's martial arts progress, while Yang Guo's Heavy Iron sword is a later one. I believe that this is ENTIRELY wrong. If we analyze the text clearly, we see that Dugu 9 Jian is definitely a later progression of Dugu Quibai's martial arts.
I will start by analyzing what is said in Dugu Qiubai's own words about the different stages in his sword training. From Shendiao Xialu:
The first sword he describes: 凌厲剛猛，無堅不摧，弱冠前以之与河朔群雄爭鋒。 is ferocious, aggresive, and able to cut through anything. It is very obvious that in this stage, Dugu was still reliant on techniques, and to an extent, the sharpness of his sword. With a very sharp sword, the opponent will naturally be much more disinclined to clash head-on, for fear of having their own sword broken. This is similar to how Miejue wielded the Heaven Sword.
The second sword he describes as: 紫薇軟劍，三十歲前所用，誤傷義不祥，乃棄之深谷。, the only clue we get is its name, Purple Rose -flexible- sword. Other than that, he just says he injured a righteous man and was deeply affected by that. We learn that in Dugu's 2nd stage, he has shifted to using a -flexible- sword. The Chinese -flexible- sword is a type of sword whose blade bends easily. Thus, it is not reliant on pure force and cutting power, but much more on subtle Yin style techniques. But by all evidence, Dugu was STILL reliant on techniques in this stage, we have no reason to believe otherwise.
The third stage, Yang Guo's stage, is: 重劍無鋒，大巧不工。四十歲前恃之橫行天下。 translates as "heavy sword, blunt edges, ingenuity in simplicty". This style is shown by Yang Guo throughout Shendiao Xialu, using simple, effective strikes with abundant inner power. But the fact of the matter is, this stage STILL had techniques and forms. However simple those moves were, they were still regimented.
The fourth stage: 四十歲后，不滯于物，草木竹石均可為劍。自此精修，漸進于無劍胜有劍之境。 Dugu states that after 40 years of age, he no longer carried a weapon. Straws of grass, branches and leaves could all be his sword. After that, he slowly progressed into the stage of overcoming the sword without a sword. This points to the fact that even in the fourth stage, Dugu STILL relied on techniques. He didn't need a weapon, anything could be his weapon, but that doesn't mean he didn't have techniques. It's just that he could execute his techniques with ANY weapon. All that mattered was the intent of the move. His fourth stage is basically realizing that sword intent is what's most important.
This suggests that Dugu obviously had a fifth stage as well like he hinted, the stage where he overcomes the sword without a sword. The important thing we have to remember is that Dugu Jiujian is NEVER mentioned. Shendiao Xialu was written well before Xiao Ao Jiang Hu, so Dugu Jiujian must have been an idea that later popped into Jin Yong's head. So unless Jin Yong revises Xiao Ao Jiang Hu and puts in a CLEAR connection point, all we can do is speculate based on the facts given.
If we now delve into Xiao Ao Jiang Hu, we learn that Feng Qingyang has mastered a style called Dugu Jiujian, as passed down by Dugu Jiubai.
獨孤九劍，有進無退！招招都是進攻，攻敵之不得不守，自己當然不用守了。創制這套劍法的獨孤求敗前輩，名 字叫做求敗，他老人家畢生想求一敗而不可得，這劍法施展出來，天下無敵，又何必守？如果有人攻得他老人 家回劍自守，他老人家真要心花怒放，喜不自胜了。
"Dugu Jiujian, only advances never retreats! Every move is an attack, attack the opponent so that he has no choice but to defend, then of course the user himself will have no need to defend. The person who created this set of sword skills, Dugu Qiubai, was named 'seeking a loss'. In his entire live, he craved only a single loss yet was left unfulfilled. Once his sword was unleashed, he had no match under heaven, what need was there to defend? If only someone was able to make him defend, he would've been overjoyed."
So according to Feng Qingyang, this set of sword skills was created directly by Dugu Qiubai, as it encompasses his style of not needing to defend at all because of his superiority. I can see how some people may make the connection that this is similar to Dugu's first sword stage as described in Shendiao, but I have to point out several differences. It is very clear in the Shendiao passage that his first stage, he was heavily reliant on superior WEAPONRY to overcome his opponents. He didn't even seem very advanced in swordsmanship techniques. Dugu Jiujian as described by Feng Qingyang, overcomes the opponent through breaking and countering the opponent's techniques, and eventually, overcoming techniques with no techniques.
Although we get a vast description of the nine different techniques of Dugu Jiujian, it doesn't mean specifically that the style is constricted to these nine techniques. While compared to Dugu's words in Shendiao, we learn that he doesn't achieve "overcoming the sword without a sword" until AFTER at least his fourth stage.
Yet, we already see glimpses of Dugu's fourth stage in the teachings of Feng Qingyang, based on his experiences with Dugu Jiujian. When Lingwu Chong is seized by Tian Boguang and his sword is knocked out of his hands, Feng Qingyang advises him:
"Idiot! Your finger can be used like a sword just as well! That move 'Treasure Filling the Room', must it be executed with a sword?"
We see here that even Feng Qingyang has broken through the rigidity of weapons and seen through that all moves are essentially the same, all that matters is the INTENT of the move. This is extremely similar to Dugu Qiubai's fourth stage as described in Shendiao.
We get further evidence in Feng Qingyang's words later:
他們不懂得，招數是死的，發招之人卻是活的。死招數破得再妙，遇上了活招數，免不了縛手縛腳，只有任人屠戮 。這個活字，你要牢牢記住了。學招時要活學，使招時要活使。倘若拘泥不化，便練熟了几千万手絕招，遇上 了真正高手，終究還是給人家破得干干淨淨。
"What they don't understand is that, the techniques are static, but the person is dynamic. Static moves can be broken and countered easily, but when faced with a dynamic user, the opponents will find themselves tied and helpless, left to be slaughtered by the enemy. So you have to remember the word, dynamic. When you are learning techniques, learn dynamically, when you use techniques, use them dynamically. If you learn stubbornly, even if you mastered thousands of techniques, if you were to face a true master, he would break and counter each of your techniques easily."
Feng Qingyang later goes on to explain the theory of "Overcoming technique without a technique".
獨孤大俠是絕頂聰明之人，學他的劍法，要旨是在一個悟字，決不在死記硬記。等到通曉了這九劍的劍意，則 無所施而不可，便是將全部變化盡數忘記，也不相干，臨敵之際，更是忘記得越干淨徹底，越不受原來劍法的拘束 。
"Master Dugu was a very intelligent person, the key to mastering his sword technqiue all rests on 'realization', it is pointless to strictly memorize the moves. Once you have become entirely familiar with these set of moves, proceed to forget all of them entirely. When encountering an opponent, the less you remember the original moves the better. You must be entirely free of the constraints of the original techniques."
We see from this passage that the main focus of Dugu Jiujian was definitely to train the user to move into the realm of overcoming technique without a technique. Further evidence from later in Xiao Ao Jiang Hu, when Linghu Chong faces Yue Buqun. This is spoken by the narrator:
"The 'Nine Swords of Solitude' that Linghu Chong had learned was formless to begin with, based on the opponent's techniques, it would reciprocate. If the opponent only had a single stance, then he would have only a single stance. If the opponent had thousands of technqiues, then he would have thousands of techniques."
We see clearly that Jin Yong treats Dugu Jiujian as a formless martial art, precisely what Dugu Jiubai was pointing to in his fifth stage of "Overcoming the sword without a sword". There is no way that Dugu Jiujian is part of any of Dugu's earlier stages. Based on the evidence at hand, I'd say that Dugu Jiujian was one of Dugu Qiubai's creations between his fourth and fifth levels.
The last question we have to analyze is exactly, where did Dugu Jiujian come from then?
I see three possibilities:
1) It was made by Dugu prior to his exile in the valley, and passed on in Book form, or to a disciple who kept a VERY low cover until XAJH.
2) It was made by Dugu during his stay in the valley, passed on in book form, or to a disciple.
3) It was made by Yang Guo.
For 1), this would be the most LIKELY case. He could've wrote down all he learned before retiring to the valley. The only problem with this theory is, why would he not keep the manual with him in his grave? It would make sense if he had taught a disciple, but the chances of that are slim as well, since this person was practically invisible until XAJH's time. Plus, no mention of this is ever made in all the carvings in the valley.
Number 2) makes a little more sense in that the manual was created during Dugu's stay in the valley, and stashed somewhere. But this doesn't make sense either because the Condor never pointed it out to Yang Guo, and Yang Guo never found a manual there.
The last scenario is that Dugu 9 Jian was created by Yang Guo later in his life and then attributed to Dugu Qiubai. This would naturally assume that Yang Guo eventually reached the stage of formlessness as according to the theories in Dugu Jiujian. However, this scenario seems unlikely as well, because Yang Guo was a very well known hero. It wouldn't make sense that his name would've been forgotten, because Feng Qingyang mentioned only Dugu Qiubai as the creator.
All in all, it is very vague as to just how Dugu Jiujian re-emerged in XAJH. What we can conclude is that Dugu Jiujian is definitely not one of Dugu Qiubai's earlier stages. All the evidence points otherwise. The most likely explanation is that this is a small plot discontinuity that Jin Yong overlooked. Perhaps he will give us the explanation in a future revision. Otherwise, all we can do is speculate as to just how Dugu Jiujian was created.