Csdrhrt

Requirements:

• One large hollow rock


• One dwarf


• One large pair of pliers


• One tunnel


• One loud voiced announcer of who is about to try to pull the sword out

Tunnel under the rock for your dwarf to get access, make sure he knows the name of the eligible person to release the sword for when the loud voiced announcer calls it out. All you need now is a mechanism for holding the sword in place while the dwarf is off duty.

“Nah, someone pulled a sword out of a stone,” said Nobby.

“How’d he know it was in there, then?” Colon demanded.

“It...it was sticking out, wasn’t it?”

“Where anyone could’ve grabbed it? In this town?”

“Only the rightful king could do it, see,” said Nobby.

“Oh, right,” said Colon. “I understand. Oh, yes. So what you’re saying is, someone’d decided who the rightful king was before he pulled it out? Sounds like a fix to me. Prob’ly someone had a fake hollow stone and some dwarf inside hanging on the other end with a pair of pliers until the right guy came along—”
Men at Arms, Terry Pratchett.

'Lock' the blade in place

Give the sword blade a fuller, the groove used to lighten some blades, and end it before the tip of the blade. It can then be inserted into a well crafted, close fitting, slot in the stone and a 'key' used to fit into, and interfere with, the fuller holding the blade in place in the slot.

Inside the stone there needs to be some mechanism to release the key holding the blade. In medieval times they had crossbows with heavy draw weights held by a 'nut and lever' mechanism. A modified version of this could be used, with a remote cable based release, to allow 'selective' drawing of the sword. Due to the strength and design of the locking mechanism no matter what force is applied the locked blade will not come free, actually the harder you pull the stronger the locking force.

It could even be made to self reset so that if the new King replaces the blade it again locks into place and again no-one else can remove the sword until the 'One True King'(TM) makes a second attempt.

Seal the sword in limestone

Limestone dissolves in acid, which medieval peasants likely will not know. Fortunately our sword-puller does. Via some mechanism or his own doing, vinegar can be poured onto the limestone to dissolve it and weaken the hold on the limestone enough to wrench the sword free. It will bubble, however, so the limestone may need to be below a layer of decorative stone.

As some display they may take a swig then pour it onto the sword. Or maybe trigger some hidden pipe below the surface.

Edit: The dissolving reaction will not remove all of the stone, only enough to weaken its grip on the sword.

The stone has only to be the cover of a tub filled with a dilatant liquid.

A dilatant (dī-ˈlā-tᵊnt) (also termed shear thickening) material is one in which viscosity increases with the rate of shear strain. Such a shear thickening fluid, also known by the initialism STF, is an example of a non-Newtonian fluid. This behaviour is usually not observed in pure materials, but can occur in suspensions.

An example of dilatant is oobleck, a mixture of water and starch:

A person may walk on a large tub of oobleck without sinking due to its shear thickening properties, as long as the individual moves quickly enough to provide enough force with each step to cause the thickening.

The faster the candidate will try to pull out the sword, the more the liquid will hold it in place. Just give the surface of the blade adequate roughness.

Just instruct the chosen one to sloooowly pull out the sword. "Patience is the key!"

Starch was a luxury good in middle age, used for starching clothes. Since you are going to pick a king, I see no issues in using a luxury good.

The sword looks like this:

The parrying hooks can be inserted below the stone's surface, where they lock into whatever locking mechanism you desire.
Thus, anyone simply pulling on the handle will be unable to move the sword.
Only the correct sequence of movements, such as pushing or turning the sword, can then release the lock, allowing the blade to be pulled out.

With some practice, these movements could be performed such that they seem natural to observers. Alternatively, with the right “ritual instructions“, not even the Chosen One needs to know about the trick.

One minor disadvantage though: The stone cannot fit perfectly to the blade, as some space must be allowed for the parrying hooks. However, this can easily be explained by saying that obviously these grooves were made by the same hooks when the sword was pushed into the stone in the first place.

Even better: This mechanism can be made fully reusable, establishing a tradition of choosing the royal successor this way over multiple generations.

Forge a blade with a very slight reverse taper, so that it's ever-so-slightly wider at the tip than at the hilt. To the casual eye, the blade should appear straight and constant width, but should be just enough wider at the tip for your blade trap to hold it in place.

Now, choose a stone with a very rough texture. The reason for this is you're going to split the stone in two, preferably in an uneven and non-obvious pattern, and you don't want the seam to be noticeable. Inside the split, carve your grooves for the blade. You'll need to be very precise as the sword should fit very snugly into the grooves.

Finally, you will need to sand off a very thin layer on the cut side of one or both halves of the stone, so that they lean towards each other, applying pressure on both sides and trapping the tapered blade between them.

Now the trick: In order to remove the sword from the stone, you will need to place one foot on each half of the stone and push them apart. If they are large and heavy enough, the stones should barely need to move to release the pressure. To the untrained eye, it will seem that your arms are doing the work of pulling the sword upward, but in reality, it is your legs pushing the stone halves apart that free the blade to be easily drawn.

To discourage other people from mimicking your stance, your "king" should be wearing a robe that covers his feet, and the stone should have naturally comfortable obvious footholds that are both on the same half, so that people will not think not to use those but instead must stand in a less obvious and less comfortable position in order to push the stones apart.

As with any good "magic" trick, some practice may be required to make the act seem convincing. A little showmanship and slight of hand (or foot) goes a long way if you know what you're doing.

The sword is placed in a scabbard. At the bottom of the scabbard, there is a spring loaded push pin lock - when the sword is pushed all the way down, it pushes the pin back until it reaches a hole in the sword, then it springs back and locks the blade in the scabbard. The scabbard has a thread on the outside.

You pour a large concrete block or you pick a large rock and you drill a threaded hole. Concrete might be easier since you can create a metal jacket that you can set in the concrete. You tighten the scabbard into the rock.

Threaded fasteners were unknown in the medieval world, nobody would think to rotate the sword, they'll all try to pull it out.

Whoever is rigging the selection can reinforce the notion that the sword is meant to be pulled out by spreading rumors among the populous:

"Surely lord Clegane will be the next king! He is so strong he'll pull it out with one hand!"

"Lord John's servant told my wife's aunt the lord is drinking bull's blood. His witch told him it will give him the strength to pull the sword out and become king!"

Then reinforce it some more with some official announcements. Have the criers announce:

"Thou who pulls the sword out by his bare hands shall have the strength to be king!"

Give this a few months to work, then announce the contest is open. Have the soldiers overseeing the line of candidates give the following instructions to each man:

"Hurry up peasant! I don't have all day! You know the rules - you have 3 pulls - hurry up now, there are 100 more like you waiting!"

Feeling rushed and excited, the peasants have no change. Then you give some of the dumber aristocrats a go - again manipulating them to use brute force.

"Lord Stark! You were glorious at the battle of Sherlock. No man is as strong as you! Give it all your might and surely the stone will yield to you!"

Obviously, while still in front of witnesses, the nobles will be given more time and privacy for their attempt. Make the circle around the sword where people are not allowed bigger - say 50 ft. When your candidate of choice is to attempt, he'll be far enough that nobody will be able to see how exactly he pulls the sword out - but still close enough that there will be no question that he did gain the sword by himself.

A different idea.
There is a release mechanism embedded in the sword hilt.
It basically has a hook that comes out from the side of the blade of the sword, blocking it inside the stone, and which can be triggered somehow.

Using a trigger is too risky: people could notice it and even realize that using it they can extract the sword. But it could use a kind of lock. I have two proposals

- The Chosen has a ring that can act as a key: it has an opportunely shaped extrusion that can be inserted in a lock inside the hint, which allows to release the hook that forces the sword inside the stone

- The Chosen has a ring made of magnetic iron. The mechanism in the hint has a kind of cavity with a metal rod, which is is attracted and moved upward, this way unlocking the hook. This last one is probably less suitable, beacuse the mechanism would need to be light in order to allow for the magnetic ring to attract the iron, but in this case it could be too fragile and easy to break if enough force is used to pull the sword

Or, if you like a less serious plot twist (even if it was explicitely forbidden by the OP): the sword is in reality a screw, and the Chosen is the only one who knows that he hasn't to pull the sword, but only unscrew it... :)

Without looking at all the other answers here. It could be possible to place a sword into a stone that is melted like lava. I imagine the stone was not brittle.

All one would have to do is add sand to liquid iron or bronze enough to give it a more Stoney texture and appearance. The differences in temperature from the sword and molten Stone would be enough to keep the sword from adhering to the stone on a molecular level.

The best time to pull the sword what day wallet it was in the middle of winter maybe on a unusually cold day when nobody is willing to come out and try. The best time to have a sword pulling contest would be in the middle of summer.