Let's return to the original text...
If the shadow of a character falls upon a ghost-stone, that person is trapped. The individual can still fight and cast spells, but is unable to move more than 10' away from the stone. The ghost-stone uses the shadowy connection to drain the victim's blood, doing 1-20 hp damage per round automatically.
Since ghost-stones look like any other stones, they normally attack with surprise. The first indication of trouble is when a character's shadow gets automatically trapped.
Any character who strikes the ghoststone physically, without first choosing an attack angle that guarantees that his shadow will not fall upon the stone, could end up trapped also. Each person in addition to the first gains a saving throw vs. wands to avoid entrapment. Only one person at a time will be drained of blood, however. Other trapped individuals are simply unable to move away. If the ghoststone survives attacks against it, it eventually drains all of its victims of blood.
So, in summary, the victim is trapped by its own shadow, which "streches" to 10 feet.
Here's a precedent we might be able to (heavily) modify...
Shadow Strands (Su): At will, a darkweaver can create an aura of weblike, supernatural, semisolid shadow that emanates outward from it in a 60-foot spread. A darkweaver can see normally through the shadow strands, but other creatures have great difficulty doing so. Each full 5 feet of shadow strands grants one-quarter concealment. Thus, a creature in the shadow strands 10 feet away from another creature would have one-half concealment and a creature in the shadow strands 20 feet away would have total concealment. A creature in the shadow strands has no concealment from creatures within 5 feet. Creatures with darkvision suffer these effects, and the shadow strands even baffle blindsight, but true seeing allows a creature to see normally through them.
Any creature that enters this area can freely move toward the darkweaver, but any attempt to move in any other direction is resisted by the semisolid shadow strands. A creature attempting to move in such a fashion must make a Strength check or an Escape Artist check; the creature can move away from the darkweaver 5 feet for each full 5 points by which the check result exceeds 10. If a creature is adjacent to the darkweaver (within 5 feet), it can move freely so long as it remains adjacent to the darkweaver. A creature protected by a freedom of movement spell is immune to the constraining effects of the shadow strands.
The darkweaver can move about as it wishes and bring the area of shadow strands with it. Creatures are not affected by the darkweaver's moving the strands, but the direction toward the darkweaver will likely change.
Creatures within the strands do not need to guess in which direction the darkweaver lies; it becomes immediately obvious when they try to move away and the shadow strands snarl and twist to block all other routes. When a creature without freedom of movement attempts to move against the strands, the darkweaver is immediately aware of its location even if it cannot see the creature due to invisibility.
Darkweavers are immune to other darkweavers' shadow strands and can see through them.
Some other possibilities/considerations:
- If a creature casts no shadow, I'd assume it can't be trapped
- If light conditions change to total darkness or other areas where a shadow isn't cast, all victims could be freed
- Rather than limiting movement to 10 feet, shouldn't it be based on the creature's Space instead?
- Could a creature with the ability to hide in its own shadow be able to hide from the ghost-stone, or would it become more vulnerable?
- How would shadow jump work?