The Immovable Sword
One of my favorite things to do when creating magical items is to take a familiar item out of the source books and re-purpose it while leaving the original function intact. The Immovable Sword in this case is exactly what it sounds like: a sword that also functions just like an immovable rod! Fun!
The utility of an immovable rod by itself is a thing of awesomeness, but imbue a different kind of object with that kind of power, and now you can start shaking up the formula.
Normally an immovable rod has a button to activate it’s properties. Here would I give the sword a magical activator word instead of a button and add a small magical bonus. A +1 or +2 bonus would be plenty for any level of hero. The utility this sword would have to a creative player should be substantial, and would be worth keeping regardless of such a low bonus.
This could also be interesting as a boss’s blade, and the crown prize for his defeat. A powerful boss could use the weapon and have access to all the utility that a player would have and create a very challenging encounter. But also consider if the heroes manged to figure out the magical activation word while they fought? Such a battle could easily turn from serious to comical pretty quick. It would be worth the price of admission I think.
On the technical side of things though, I would probably try to devise a way to limit “spamming” the activation word in a round of combat. Perhaps the use of the activation word could take the round’s reaction and/or swift action (5e D&D). That might limit the player from abusing the item. It will give them meaningful choices if activating the power takes the place of an attack of opportunity or something. Tailor your limitations as you like according to your chosen system. But do include a limitation so you won’t have to add one later ad-hoc
One obvious quest idea for this item is the ol’ “sword in the stone” story. With your own D&D twist of course.
Maybe a devious wizard set up the challenge offering a wondrous and very expensive prize to the one who succeeds in pulling the sword from the stone (a prize he doesn’t have or is unwilling to give). But he charges the witless townsfolk to make an attempt, knowing they’ll never succeed without the magic word. He pockets the coin to fund his lifestyle or research. What could happen if the heroes discovered the secret and exposed the wizard for the scoundrel he is? Or perhaps they helped a villager claim the prize after discovering the secret. The quest would likely revolve around a showdown with the wizard refusing the prize and retreating to his lair.
Perhaps, as a different scenario, the sword has been fixed in place for hundreds of years and the reason has been lost over time. Now the villagers believe that the one who can retrieve the sword is the chosen champion of a deity. Maybe it’s a deity that is opposed to the beliefs of the heroes, or evil. All kinds of misunderstandings can be precipitated from whatever myth you devise for the mysterious Sword. This scenario will be far more interesting if the heroes acquire the sword before learning of the villagers’ superstition and then has to deal with their false impressions.
I’ve never used this idea yet (and probably can’t now), but it never hurts to keep it in mind as a surprise addition or quick quest idea.