From Edge of Darkness Wiki
Soul Stones represent the distillation of a mage's connection to his Path and the Supernal Realms into a temporal, physical form. Soul Stones may not always appear as stones, they may take the form of flowers or plants or animal tokens. Soul stones require a sacrifice on the mage's part, solidifying a piece of his soul into physical form, this requires a powerful exertion of will, as the soul will instinctively fight against the act. In addition to the immediate sacrifices, a mage's future potential is constrained, as one cannot fully develop their connection between the Supernal while the soul remains fragmented.
One of the most well known benefits of creating a soul stone is the capability of creating a Demesne, or a region brought into resonance with the Supernal Realms. Demesnes often take on aspects of the Supernal Realm to which they are tied, allowing vulgar spells cast within the Demesnes to be covert. 
The creation of a soul stone requires a conscious and genuine desire to distill a piece of their soul into a physical form. The necessity of desire means that a mage cannot be supernaturally compelled or forced to create one. The act is traumatic and (sacrificing a dot of Willpower) personal enough that it must be done fully accepting the consequences. The act of creation can take the form of a period of meditation, a sojourn to the Oneiros to inspect elements of the soul that may reflect in a soul stone, or the instantaneous focus and desire for the temporal power they provide.
No roll is required. The player sacrifices of a dot of Willpower, and spends one point of Mana. Soul stones take some form related to the mage's Path. The Sacrifice of mystical potential for temporal power is a sin against Wisdom 4. Additionally, each soul stone created reduces the maximum Gnosis a mage can achieve by one. With the first soul stone the mage's maximum Gnosis is 9, if she's created two soul stones her maximum possible Gnosis is 8, etc.
For a mage to attempt to try and create a soul stone with a specific shape, they may attempt to create a physical facsimile of the intended form, the mage then rolls Wits + Occult as the soul stone is created. If materials and designs foreign to the Path are used, penalties to the roll may apply. If successful, the soul stone is guided to the intended form, failure on the Wits+Occult roll means the mage failed to account for the soul's hidden desires and the soul stone governs its own form.
It is impossible to deny the benefits that can be gained via the creation of a soul stone. Although not without risks, the creation of a soul stone has served as the foundation for many powerful mages and cabals. Demesnes, allowing the casting of far reaching but vulgar magics; magical tools that can be used from hidden areas; or as the basis of powerful Imbued items.
Demesnes may express influences of the Supernal Realm of any soul stone used in its creation. No more than five soul stones can be combined into a Demesne at one time, The area encompassed by a Demesnes depends on the number of soul stones included in its creation, and is commensurate to Sanctum Size.
Within the area of a Demesne, all spells are initially considered covert in aspect. Even normally vulgar spells are protected from inducing Paradox, although the spells are not visually altered. A fireball flung from finger tips still flare and burn through the air, or a telekinetic sword swings disembodied through the air. For this reason mages must still watch for Sleepers within Demesnes, as flashy spells quickly become Improbable.
In addition to all spells being covert in aspect, spells that include the Ruling Arcana of the Supernal Realm reflected (Forces/Prime for Aether, or Life/Spirit for the Primal Wild) receive a +1 bonus to spell casting dice pools. The area within a Demesne also takes on an atmosphere resonant with the Supernal Realms invoked.
- Forces and Prime Spells gain +1 die bonus
- Static electricity is common, even in high humidity. Fires may spark easily
- Objects seem limned by a dim glow or aura.
- Fate and Time spells gain a +1 die bonus.
- Improbable events occur more regularly
- Everyone has an impeccable sense of time. Knowing what time it is, and how much time has passed.
- Mind and Space spells gain a +1 die bonus.
- Memory recall is sharper and quicker.
- People have strong senses of direction, navigation around the area is easy, even in low lighting or smokey rooms.
The Primal Wild
- Life and Spirit spells gain +1 die bonus.
- Minor scrapes, bruises, or aches are felt less acutely.
- The condition of all things affects the atmosphere. (Good and healthy provides +1 to Social rolls, pool condition or sick -1 penalty).
- Death and Matter spells gain +1 die bonus.
- Shadows are deeper, or more stark.
- Objects seems solid, less likely to break.
Dedicated Magical Tool
Soul Stones can be dedicated as magical tools. They are dedicated in the same way as any magical tool, which in and of itself wouldn't make it worth the cost of creating the soul stone. The greatest benefit provided by a dedicated soul stone is that it may be accessed without direct contact. A dedicated soul stone may reduce the risk of Paradox for its creator while within a short distance, no greater than that created by a one stone Demesne. 
Soul stones, resonating with both the Supernal Realms and the soul of the mage, can more easily receive imbuements. When a mage uses a soul stone to contain an imbued spell, the spell automatically receives a +2 to Duration factors. So the base duration of the spell is 12hrs, or 2 days with advanced prolongation factors. In addition, spells that include the Ruling Arcana of the mage's Path gain a +1 bonus on any activation roll. 
|This provides a +1 Potency factor to the imbued item spell, as applies to the dice pool for activation.|
Akin to the function that can be served by a Demesne, mages may use their soul stones as a focus that carries even greater power than more common magical tools. By using the soul stone as a focus, a single vulgar spell based solely in the mage's Path can be cast as if it were covert. This requires an instant action of concentration, which occurs after the turn in which power is built using High Speech, and a point of Willpower. This "portable Demesne" effect can only be used once per day, per soul stone, and risks burning out the soul stone. After the spell has been cast, roll the mage's Gnosis. A failure causes the stone to go inert for 24hrs, during which time it cannot be included in a Demesne, serve as a magical tool, be imbued, etc. A dramatic failure on the Gnosis roll causes the stone to be inert until another point of Willpower is spent, after which the stone is inert for another 24hrs before becoming functional again.
Creating a soul stone can be beneficial to the mage, trading Supernal Potential for temporal power. Distilling a part of the Awakened soul into corporeal form presents certain risks as well.
As noted above there is the creation of a soul stone is an act of hubris, requiring a degeneration roll of 3 dice, equal to a sin against Wisdom 4. This is comparable to forcibly draining Mana, or otherwise limiting supernatural potential. 
Each soul stone created by a mage restricts the maximum to which a mage may advance her Gnosis. The maximum Gnosis a mage can reach is equal to 10 minus the number of soul stones she has created. This lost potential can be regained by reincorporating the soul stone back into mage's soul. This can be done at will, but requires the sacrifice of a dot of Gnosis. Gnosis can then be increased by the expenditure of XP as normal, at the regular cost of New Dots X 8 experience points. There is a great cost associated with the temporal power associated with soul stones, and restoring the lost potential comes at an equally great cost.
As a piece of a mage's soul, and connection to the Supernal Realms, a soul stone may be used against a mage should it come into the hands of an enemy. It is common practice in many Consilium that when a mage comes into possession of another mage's soul stone, the possessor may demand three services of the soul stone's creator, who is termed a Thrall. The soul stone counts as an intimate sympathetic connection to the Thrall, and allows the mage to access the Thrall's Mana stores once per day, with a roll of Presence + Gnosis - the Thrall's Resolve. This counts as one of the favors allowed by possession of the soul stone.
The soul stone is a piece of the mage's soul made manifest. As such it is vulnerable to physical attack, though they are resilient. Soul stones may only be damaged by aggravated damage, and every soul stone has a Durability equal to its creator's current Gnosis (`not` the Gnosis at the time of its creation), and a Size of 1. The incredible durability of the soul stones of senior mages means that soul stones can live on well past their creators, and this longevity also applies to Demesnes based on these relic soul stones.
Should a soul stone be destroyed, it is lost forever. The mystical potential bound up in it is lost, never to be restored, except potentially by an archmaster.