Legendary weapons are just common weapons made with ordinary material we have known for, some are made from iron or steel, other are made from stone and durable wood. Weapons that can shatter land or cut the mountain or other miraculous feats, I will call those nonsense, unless those people incorporate nuclear or bomb into their weapon, then again it will no longer just merely weapon it's machines. Some of those weapons were sharp but does it stay sharp, some of it were durable but it will likely broken as time passing by. What make those weapon legendary was the person who wielded it and preserved into lore, tale and story. I remember one story where MC tried to find legendary sword, after found it he discover that sword already corroded and broke after several swing hahaha . Sorry, I didn't like how people mentioned legendary weapons and old artefact are the best tools or devices, it might be one of the best on it's time but human always advance in technology and people will invent better tools than previous ones.
Waaaaah Enough with the Dragon this Heaven that, Moon Goddess, and insert-color-Emperors already Btw wth are these generic colored Emperors even called that if they're usually snivelling brown-nosed minions of some other dude?
why not make the sword a super OP in the beginning like severing the existence of everyone (I mean if they are cut by it their whole existence, like their past and their future be annihilated) and then make a super strong enemy like a God in the world and then the protagonist will use it against that enemy and die with the enemy (but the enemy counters the effects somehow) and then they will reincarnate at same time and they will cultivate like crazy and they will try to find where the sword is again and they will fight for it.
It is rather well known that humanity has wielded steel for a lot longer than previously expected. In fact some steel can be traced back to the early iron age and even the bronze age. Now imagine what a steel sword does to a bronze one... yeah. Destroys it in a single hit, and then goes trough the armor as if it was wet paper. Very likely these magical swords were exactly that: Steel weapons in an age when the rest of the world was still stuck with trash. So, with that out of the way we can focus on what legendary weapons are. First off it has to be... legendary. It needs a history. A weapon like this might go trough many hands, each adding to the legend, as it is used to cut down hundreds of enemies, or maybe just one particular type of enemy. Imagine a clan of demon hunters handing down a blade trough the generations, or a king doing the same with the symbol of his power. It also very likely features a special effect, or an active use that is rather potent. These weapons can be either cursed or magical in nature. Does it devour souls to empower its wielder but destroys him if it isn't fed regularly? Or does it sever the immortality of an otherwise highlander-like existence? It might also give the wielder the lifespan of his slain foes, or grow stronger the more blood it drinks. Or it might be a sharpened stun-gun on a stick. Who knows.... But you want weapons which are more famous/infamous than their wielder. That means they have to add more to the battle than them. Period. If an object is supposed to outshine a character then it needs to be one itself in a sense (even if it is not sentient.) It creates its own legend as time passes, regardless of the outcome of its battles. Is it a treacherus blade? A sentient/intelligent one? tl;dr: Nobody cares if someone wields cheesecutter the IV.. But they would care if it was Excalibur (not the sword in the stone btw) or Balmung. This will be at the cost of your characters tho.
In anime and shounen manga weapon are more of a reflection of character. Bleach (as much as I don’t like the world building and story) is an excellent example of this. Most MCs use a sword (or katanas; there are a lot of katanas) because they reflect a honest or noble character and even kingship. If they character is sly they might be using needles, wire thread or other unconventional weapons. If they are sadistic it might be a whip of a scythe. The super powers that these weapon have also reflect these themes and character traits. If you are already familiar this all this stuff and you just want some ideas of superpowers attach to the weapon start with a verb. (Ex: the weapon can sew. It can sew a shadow to the ground. It can sew people back together. It can sew a soul back to a body.) Add themes to give it flavor. The basic Fire, earth, wind and water are popular ones. Shadow for the example I used above. You can get some fun or weird results. Hopefully that made sense (I get the feeling it didn’t.)
Whatever you do, don't make it sword. They're so vanilla. I recommend looking through stories of myths and legends and use them as a base to create your weapon. Granted, they've got a lot of swords as well, but there's also the occasional talking mace, magic arrows, badass spears, tridents, and a literal thunderbolt. You can mix and match the attributes to your liking.
Make them part of a set, a single weapon can get boring easily but the interactions between weapons and their wielders can be cool Have the weapons have a cost, a part of the user is very common but also too common, requiring "ammunition" is also cool, like an axe that can get on fire, teleport back to the wielder and release a cutting wave, but every use of every ability consumes a portion of a certain material Interactions with the world, weapons that are good or bad in certain environments or for certain fighting styles, having an allmighty weapon in boring
As a reader, I am always drawn with that Mysterious Weapon that is forgotten by mankind. Mysterious in a sense that it has an intrigue in itself that is relevant to the plot. The weapon being a missing piece in the puzzle. Or... maybe a spiritual tie with the weapon? I always fall for the trope where the weapon acknowledges you to wield it Unique Legendary Weapon. There other legendary weapons, I assume. So maybe it has a special technology that is from an unknown time period in comparison to the Mainstream Legendary Weapons from Glorious Era? Does a talking weapon count? XD Lol. Just my four cents...