This spell requires two solid stones.
The stones must be the same weight, as closely as can be measured by the Magic-User when the spell is cast.
The two stones must be of a different kind, at a base enough level that there is no easy argument to be had about their difference. Some dust their stones with pigments, others use different quarries.
The stones may be changed. They can be polished, scraped, chipped, and chiseled as much as is needed. A pair of sculptures could be used as easily as two natural specimens. They may be drawn upon, marked, and altered in any imaginable way. Things may even be attached to them. As long as their weights are matching when cast, all is well.
The location chosen must be known to the Magic-User, but they do not need to have seen it. As long as the image they have of this place in their mind is true the spell will work. If it is something they see now it is irrelevant if that place is in motion, or changing in other ways. It can be living. Distance is meaningless. With clarity of mind any site is possible, if the mind sees it true. A faulty mental image is, however, a hazard.
When this spell is cast the two stones must also be known by the Magic-User, with the same lack of limitations on distance and the same correctness of location required.
The spell takes effect instantly. One of the stones, as chosen by the Magic-User, is the Stone-in-Motion. It travels in as straight a line as can be drawn to the chosen destination. It can change direction instantly, with no loss in velocity, if this is needed.
It travels at terminal speed, never stopping or slowing. It never makes any contact with things strong enough to harm it and end its travel. All else is plowed right through.
It can come as close as an eyelash is thick without touching, but when it reaches its destination it touches it with full impact, as if dropped from the top of the sky.
The second stone is the Inert. It must remain in exactly the same place as it was when the spell was cast. As long as it does not move in any perceptible way the Stone-in-Motion will continue movement until reaching the chosen destination.
If the Inert Stone is moved, the Stone-in-Motion will instantly comply with physics and plummet down as demanded by local gravity. If it has reached its destination already there is no effect, as the spell has already ended.
A living body, even one trying to be still, is always in subtle movement. The spell cannot be cast safely if the Inert Stone is being held.
The Inert Stone does nothing to indicate the Stone-in-Motion has reached the end of its path.
EFFECTS OF A MISCAST
This spell is potentially miscast if the stones are not of the same weight, the secondary stone is not truly still at the time of casting, or if the destination is not actually truly "known" by the caster. The Referee should never, ever warn an Adventurer of a potential miscast if they have not made sure themselves, as it is their responsibility to take precautions.
1. The Stone-in-Motion does nothing, the Inert stone does the same. The Magic-User is instantly flung towards the intended destination as if they were the Stone-in-Motion. They cannot be stopped, and will ultimately collide at terminal velocity.
2. Both stones become Stones-in-Motion, travel to their intended destination, and without making contact with that destination, instantly begin a linked orbit around it at terminal velocity. The stones are half the distance they traveled apart, one across from the other, and the smallest possible distance above the intended destination. They will not change their path to avoid any obstacles, and will not stop their orbit unless destroyed.
3. The Stone-in-Motion does nothing. A different stone in the vicinity is propelled instead. Roll 1d6 to determine its size. 1 is a thumbnail pebble, 6 is a boulder large enough to crush a bull. It comes from the ground below the caster. If the Inert Stone is moved its motion ceases, as usual.
4. Until the Stone-in-Motion reaches its destination, the Inert Stone and everything capable of perceiving the Inert Stone is unable to move, or be moved, by anything. This is something like being frozen in time, "paused" as if in a video from the perspective of your own agency. You do not starve, and need not breathe, but you do exist and can be tampered with. Once the Stone-in-Motion arrives at its destination a Magic Save is required to free yourself, and move again.
5. The Stone-in-Motion instantly splits into two pieces, then four, sixteen, thirty-two, and so on to travel in infinitely many directions at once. This can be like a powder horn exploding, or a barrel of gunpowder, depending on the size of the stone.
6. A piece of the destination equal in weight to the Inert Stone is detached from the whole and pulled towards the Inert Stone at terminal velocity. It cannot be stopped.