How would you handle the Legendary Sword of Plot Advancement trope to make it more interesting?

Is there anything to make it more interesting than just "Magic Boomstick that can magically damage the final boss for some reason when other weapons can't". This seems awfully contrived and boring. Whenever I think of including one in any story I come up with, it just ends up seeming like a McGuffin.

For clarity I'm talking things like the Master Sword, Falchion, and the Binding Blade, among others. How would you make it more intersting? Or can you maybe provide an example of a story that did something creative with it?

I think it is all in the delivery. If you know that a weapon can hurt a boss, I think it is helpful to explain why and have the characters engage in that part of the story. Also, attempting other options and failing can be a good way to make it feel more organic. "It isn't the legendary sword, we threw everything else at him and this is the only thing that makes him bleed. So lets make 100 of them and make sure everyone has one"

I think if it matches the pace of the story and the players have a chance to engage it isn't boring.

I do think, "Nothing can hurt him except this magic thing" with no explanation is crazy and silly. Why can nothing else hurt them? In all of magic, NOTHING can ? That seems very hard to prove or feel like it is true.

Just my two cents.

I concur with @Tagris. I think it ultimately comes down to delivery and how you introduce it. If your story needs a villain immune to everything but the Sword of Plot Advancement, we should ask ourselves why... how did he become Immune to All Things? That gives us the edge for Why The Sword?

That said, speaking as somebody who has run D&D games for 25 years, your players have a tendency to get grumpy when they face a villain Immune To All Things... It feels like a contrived plot device because you're not sure how to prevent them from logically wasting the villain too early. Where possible, I recommend making the plot support reasons not to kill the villain rather than a game mechanic.
Ideas:
Maybe he has a dead-man's switch (IE: If you kill me, you release the 20 demons of unspeakable power bound with me upon the world. Muwahaha!)
Maybe he has a hostage (IE: If I don't check in with my people once a day, they kill your children.)
Maybe he just is god-awful hard to get to (Because I'm paranoid, I've created a super-villain lair where you'll never get to me).

In these situations, you get the same result (big quest to overcome the "protection" the villain has), but it's not so arbitrary as Sword of Plot Advancement.

If you must use a game mechanic (Sword of Plot Advancement), here's some ideas:

Maybe Big Bad Villain performed some Blasphemous Ritual of Imperviousness, and the sword was his at the time.... Making it capable of Cutting Through Magical Protection. Villain has since lost sword, or sword was stolen by a prior (and failed) group of heroes, and now it's up to our party to find it.

Maybe Big Bad Villain is part of a line of deities Immune To All Things. However, as it turns out, there was some mystic quest Centuries Before Our Story to create a Weapon that Slays All... Maybe it was even created by these deities Immune To All Things as a way for them to wage war on one another... It was never meant for mortals, but alas, here it is. What else are we to do but find it and use it?

Maybe Big Bad Villain died a Horrible, but Poetic Death, and this Sword of Plot Advancement has personal significance. The reason it affects him is more spiritual than actually magical. (IE: It affects him because he believes it will, not that it has any magical edge over any other magical enchantment, pardon the pun).

Maybe Sword of Plot Advancement isn't the only thing that can damage Big Bad Villain. It just happens to be the most easily obtainable (which isn't to say that it's easy, as it requires at least a 10 hour plot arc to get it!).

Hope these help!

Your first problem is by thinking it's not interesting. If it's not interesting to you, then why bother going with it? Why force yourself to revolve your game around a concept you don't find interesting?

I, personally, don't have strong feelings about legendary weapons that are required to defeat the main villain. It's just another weapon upgrade to me, and luckily they don't involve a grind of some sort because they're given to you through the story. Bonus points if it happens to look awesome.

I thought about this one for a long time, and it isn't that it's necessary to defeat the boss; but rather to get to them. Looking at it as a symbol rather than just a means to an end puts it in a different category.

Chrono Trigger did this with the Masamune, and it became it's own entity.

FF2 Cecil had to relinquish the dark sword for a light one, and it became his entire backstory and destiny slowly throughout the game.

For my own game it comes down to the fact that the weapon was sealed away for unknown reasons, and inspires fear more than it is effective. It can also change the user because of how it's acquired in terms of the plot. The journey to get to it is interesting, and the sacrifice to have it is unique.

A legendary object usually has its own history, and in fantasy worlds, often a will of its own. These qualities can easily be used to make a sword something more than a sword. Foreshadow. Mention the sword in old legends, on ancient documents, in old people's campfire tales. Make those mentionings somewhat vague, though, and include hints to some kind of "dark side" to it (even more vague). Then, just some time before the sword is getting used, let the heroes find out that the sword gains its power by sucking out its owner's soul... or those of his friends. Let the sword speak to the hero in the dark, lonely hours when he/she sits watch, or maybe just let it convey its emotions - to show that it has its own goals and wants to seduce its bearer, subtly lead them astray. The sword might turn out to be a worse enemy than the boss...

I think I would make it so this "legendary sword" is not a literal sword, but something else. Something that the player finds out over time when they gather people for their adventuring party and get to know those people. The kind of thing that evil kings and dictators fear the most...

I've seen this concept better handled in books than video games. A few examples...

Lord of the Rings: Heroes already possess the ultimate weapon and must face temptation to use it selfishly.

Harry Potter: Villain posseses the ultimate weapon, but it will only work for the hero thanks to a trick of destiny.

Prydain Chronicles: Hero needs to learn humility, not courage and strength. Hero acquires magic weapons but his companions or rivals use them instead.

I actually tried something different on this in a game I wrote up back when I was younger. Basically, it was heavily suggested to the player that they go find this legendary sword, but, if they were very good or very stubborn, they could win the final fight without it. However, the final fight was hard enough that if you didn't have it, the odds of winning were almost 0%.

You can also go a long way with weapons getting lost, destroyed or otherwise failing.

In FFVII, Jenova cells and people who possess them are ultimate weapons. Cloud thinks he has/is the only weapon capable of defeating Sephiroth, discovers that isn't true, deals with the blow to his ego and still has to face Sephiroth with nothing more or less than his inner strength.

In my game, there are holy weapons (God weapons, actually) and even relics... It doesn't hurt just certain people, but it can kill certain people. In my game, the people are near immortal, meaning if you don't do anything stupid, don't get blown up or something doesn't interfere with your near immortality, you can live forever. These weapons, however, are connected with a bond to the God or Goddess that own them and can chose to be with someone or some where. If someone unworthy tries to weld them and do harm, they will vanish.

So, another related question I have: which of these is the better approach to obtaining the Legendary Weapon of Plot Significance?

Is it better to obtain the legendary weapon at the end of the game, having the obtaining of the weapon be the focus of the quest?

Or, is it better to give the player the legendary weapon early on, and have it grow in strength alongside the protagonist throughout the story?

Neither option is inherently better than the other, imo - it all depends on what your story is, and how you implement either option.

On the note of the Master Sword, I actually really like the reasoning behind it; It's not that the big bad is immune to everything else, but rather that the Master Sword ignores invulnerability of all evil beings. If a being is invulnerable or nearly, then the established sword is sought after to solve the problem. It's more like "Hmm, tree blocking our path? Grab the axe.".

In my own games and stories?
Legendary weapon is made from the big bad.
Legendary weapon doesn't work. Oops.
Legendary weapon is merely the simplest way to hurt them. Everything else does 10% damage.
Legendary weapon actually protects the user instead. User is immune to big bad's death attacks.
Legendary weapon isn't a plot device. It's a side quest that justifies a game breaker.
LW is merely some really nice weapon, but combined with something else (the MC, another item, a different big bad) it becomes enough to hurt the big bad
LW is the starting weapon and the MC is the hero because they happen to have it.
LW isn't there to stop the big bad, but to stop something else; It's strong enough to combat earthquakes.

I probably have more I'm forgetting.

The way I've handled the ultimate mcguffin so far is to make it a person rather than an object. Essentially the legendary weapon is an old resistance leader who was given the knowledge to operate alien technology that is so advanced that to the rest of the party he is a mage using magic by a detractor from the aliens who disagreed with how they were treating the denizens of the planet they conquered but not enough to actively fight his own family at the time, so he chose the most successful surrogate he could find to teach to fight them in his place.

This revolutionary is in turn betrayed by some of the people he is fighting for, and sealed away as a science experiment years before the game actually begins. He is rescued by the protagonist fairly early in the game, but has lost his faith in his own people after being betrayed by them for a few creature comforts and safety from being culled by the overlords and the early parts of the game becomes more about learning from and reaffirming this one betrayed man's faith in humanity so he can/will fight again than it is about finding the one tool that can definitively save the day.

The rest of the game is about raising an army to fight with, and military conflict where said man opens impenetrable doors and cancels out attempts at using advanced technologies to destroy the resistance outright, rather than fighting on their level he merely forces the invaders to fight on the level of humans at war, the one thing that we wage consistently well.

There are a lot of interesting paths you can take the legendary weapon trope, you just have to think it through and consider not just that it exists but how it shapes the narrative both before, during and after the game ends so that it feels like an organic part of the world rather than just a "success key" for solving otherwise completely hopeless situations.

On the other side of things, the Big Bad...
...might have a weakness to a material, not a specific weapon, but only one weapon was ever made entirely with the material (silver is a common weakness and hard to make a usable weapon with).
...isn't invulnerable but just too strong. The LW gives the user enough strength or mimics the enemy's strength.
...is too fast and dodges all attacks. The LW isn't detectable (and thus, hard to find).
...intentionally made the weapon. Maybe BB was once good and knew evil was possible.
...could be taken down with an army, but needs a LW to fight with one person (e.g. dragons).
...is in stasis (or sleeping) but is still able to affect the world. The LW makes BB vulnerable, but able to fight you. (FF4 does something similar)
...used the item to become invulnerable, sapping most of the LW's magic, then threw into a volcano/ocean/outer space because BB knew it would be a weakness.

Legendary socks of plot advancement?

Or you have to wield a legendary person to defeat an evil weapon.

@Frogboy and @kirbwarrior Please don't spam the thread.

I guess in a future game my "sword of future plot advancement" will be... a harem. You win the love of a LOT of girls and use that collective love to kill the final boss...