The aura may be the best approach, as there are no rules to really adjudicate where a shadow falls.
Here's a stab at a first draft...
Shadowtrap Aura (Su): A ghost-stone continually projects an aura to a radius of 10-feet that grabs hold of creatures' shadows. Any creature within the area that casts a shadow must succeed on a DC X Will(?) save or become tethered to the ghost-stone via its shadow for x rounds/minutes. The save DC is Charisma-based.
A creature trapped by the aura may not physically move (or be physically moved by others) outside the aura's area, but is otherwise unhindered. The area within the shadowtrap aura is treated as a dimensional anchor for purposes of teleportation and other forms of extraplanar escape. A creature with the shadow jump ability or other mode of shadow travel (such as the shadow walk spell) may escape the area as normal for the spell or ability.
A ghost-stone may employ its blood drain upon any creature trapped by its aura. A ghost-stone's shadowtrap aura does not function in areas of total darkness.
Hmm, here's going to be another variant. As far as teleportation, I'd rather allow a caster level check every time or not at all. The once per day bit seems too exception-based and fiddly.
I like the idea of the consecrate or hallow spells, but hallow isn't going to help much since it has a 24 hour casting time. (Consecrate is fairly expensive, too, though it at least only takes a standard action to cast.) I'm uncertain about requiring a save to exit, though; maybe it should just let the victims out.
Here's my stab at it:
Shadowtrap Aura (Su): A ghost-stone continually projects an aura to a radius of 10-feet that captures creatures' shadows. Any creature within the area that casts a shadow must succeed on a Will save or become tethered to the ghost-stone via its shadow. The save DC is Charisma-based. A ghost-stone's shadowtrap aura does not function in areas of total darkness.
A creature trapped by the aura may not physically move outside the aura's area (or be physically moved by others), but is otherwise unhindered. A trapped creature can use teleportation or other forms of extraplanar escape to leave the shadowtrap area if they succeed at a caster level check (DC X plus the ghost's Hit Dice); otherwise they are affected as if by dimensional anchor. A consecrate spell cast within the shadowtrap area will temporarily disrupts the aura, granting trapped creatures one round to escape the aura. A creature with the shadow jump ability or other mode of shadow travel (such as the shadow walk spell) may escape the area as normal for the spell or ability.
At the end of its turn each round, a ghost-stone drains blood from each creature caught by its shadowtrap aura, doing 1d4? Con damage to each. Each time it drains blood, the ghost-stone gains 5 temporary hit points.
Hmm, here's going to be another variant. As far as teleportation, I'd rather allow a caster level check every time or not at all. The once per day bit seems too exception-based and fiddly.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.