The demon of Sacred Oaths is considered to be the seven eyed demon sword, but in Swords CCCLXII-At the Dawn of Time, it is confirmed that she used to have eight eyes, but one was stabbed out by Kargob. Because the demons follow a sequential order of eyes, this implies the existence of true seven eyed demon sword that hasn’t been mentioned. I believed that we have solid evidence of its existence from the Seven Headed Wyvern and the Monster Magician, who were possessed by or bonded with the sword. First off, demon Swords have repeatedly shown the ability to inhabit hosts, typically as a way to escape isolation, in the many exploits of the Demon King along the main storyline, the dire wolf and the Demon of Absolute fury, the Sword of Relentless Omens posessing the Whitesmith, and the unexplained body for the Demon of Endless Chaos in Swords CLVI-Five eyes blue demon. The firat subject for the mystery demon is the Monster Magician. The Monster Magician is shown to the have incredible power to imitate creatures abilities, something that is so-far unique to him, and even strong enough to replicate a sword dragon. But what links him to a demon sword is his appearance. In Swords CXLVIII-Monster Magician, we get a full view of his appearance. He is colorful with spiked hair, a carapace looking undershirt, and tooth like decoration, on his hair and clothing, which is similar to the many other demon sword posessions. More important however is that his coat has exactly seven eyes on it, three on either side of the chest and one on his right wrist. Additionally, in Swords CDLXXXIX-Ivory tower, he is seen with a strange orange spirit (which bears a remarkable resemblance to sword souls from the godswords) hovering around him, confirming his connection to some kind of entity. Although less certain, this demon sword could also be connected to the Seven Headed Wyvern. In Swords CCXV-Six Sword Master, there is a Wyvern with exactly seven heads, and though its color scheme is different from the Monster Magician, it isn’t wildly so, and it is possible for the Wyvern to be the host of the mystery sword somewhere else on the time-line. We do get conformation that the wyvern is connected to a demon sword because of the secret text, which says that the Seven Headed Wyvern ate the nine eyed demon sword. This also gives evidence of a nine eyed demon sword, or perhaps this means that the sword I’ve been talking about is actually the nine eyed one, and the Monster Magician has more eyes than we can see. In ‘Monster Magician’ we see an eye on the Monster Magician’s right wrist, but in ‘Ivory Tower’ we see one on his left wrist. This could either mean that there was an accidental inconsistency, it shifted over time, or that the sword has more than 7 eyes. To conclude, I believe that a seven/nine eyed demon sword exists, and that it is posessing the Monster Magician, giving him his powers, and that it is benign/neutral like the Demon of Sacred Oaths.
This is an interesting theory, I hope we see more of it later on! 
There are some that argue the depiction of pandemonium disproves this theory, but I think that it gives ground instead. Since we have seen all 8 of the demon swords fused together, we know that Pandemonium has 8 eyes, one corresponding with each demon sword. Slotted between Unspeakable Darkness and Sacred Oaths (6 eyes and 8 eyes respectively) there is another demon eye which doesn’t match any of the others we have seen so far. It has a uniquely black eye, and the main composition of its tendril is orange (matching the general color theme I have noticed so far). Because of pandemonium, there has to be another demon sword. Maybe its name is something like “Unknowable Secrets” and part of its nature is obscurity. It seems to be intentionally quiet during the pandemonium scenes.
I have spoken much about colors of monsters that are potentially related to the true 7 eyed demon, but with the latest comic, “DCCCXIV The Big Bad Wolf” It is shown that the Monster magician is connected to a demon sword, but it is just Endless Fury, a demon we already know about. With that, I can admit that most of these monsters probably don’t have a connection to the mysterious seven eyes. However, we see Pandemonium with 8 unique eyes, and one of them is brand new (reference comment above). We know that this secret 7 eyed monster is very orange, perhaps some red, but it also has a fully black eye with a red slit. We know this must have significance because it is the only black eye, and the tag for Pandemonium shows only the black eye with the red slit. I can’t think of any connections currently, but perhaps a more certain indicator of a connection to this demon is not the color orange, but rather the presence of black eyes with red pupils. If you know of such a creature, I would be grateful to know.