Soulcasting is a form of Surgebinding on Roshar. Soulcasting involves manipulating the Surge of Transformation. A Soulcaster may refer to a special kind of fabrial which allows its user to Soulcast, a person who uses such a device, or a person who is able to Soulcast by means of Surgebinding.
Soulcasting by Surgebinding
Soulcasting allows the Surgebinder to transform any material into any other material, with the exception of gemstones and Shards. It is much easier to create the Ten Essences than other materials; for example, fruit juice would be harder to make than blood, because blood is one of the Ten Essences.
In order to perform a Soulcasting, the Surgebinder needs to send their mind into Shadesmar to connect with the Cognitive aspect of the object they wish changed. Once connected, they must convince the object to change. The process would be easier with more stormlight. Presumably, living creatures have a Cognitive aspect that is separate from their mind in Shadesmar. Lightweavers and Elsecallers have inherent Soulcasting ability.
Soulcasting with a fabrial
A Soulcasting fabrial uses Stormlight from a gemstone corresponding to one of the Ten Essences. In order to Soulcast a specific Essence, the Soulcaster needs a piece of the corresponding gemstone attached to it and infused with Stormlight. The Soulcasting then draws Stormlight from the gem. The fabrials themselves are considered holy objects in Alethkar, to be used only by ardents and in extreme secrecy. The number of essences a Soulcaster is able to transform varies, though most are attuned to a single essence. Kaza states that her Soulcaster can only use one mode, not the full three. Soulcasting with a fabrial requires the user to be in physical contact with the object they wish to transform. Soulcasters with access to the same essence can work together to Soulcast objects of greater size. Navani talks about how they should be able to create any liquid that is water-soluble with a Soulcaster that is attuned to the essence of blood using a garnet. Soulcasters are of ancient design, and how they are made is unknown.  It may be the case that the only way to obtain aluminium on Roshar is through soulcasting.
Primary Usages of Soulcasting
Soulcasters are extremely valuable for its various and powerful usages. For examle, Soulcasters are essentially what enabled large scale wars on Roshar. Maintaining supply lines is a crucial problem to any army, but with Soulcast grains, an army is able to fed more men, move faster and sustain stretched war while not worrying its supply line being attacked. Soulcasting is also very useful in engineering. One can first put up a structure with light and easy-to-construct materials, wood for example, then Soulcast it in to solid stone. In close-ranged combats, Soulcasters can kill their opponent by Soulcasting their body into anything else, though this is strictly prohibited. In medical fields, Soulcasting the blood can remove certain poisons.
The long-term use of Soulcasting, whether via Surgebinding or a fabrial, can lead to the user becoming a savant, transforming slowly into the essence they frequently transform other objects into. The effects of savanthood are much less pronounced for Surgebinders and can take a different path in their manifestation, due to the Nahel bond. Some Soulcaster ardents on the Shattered Plains developed crystalline eyes and stone-like skin with cracks. Another ardent in Kholinar appears as if there are vines growing under her skin and poking out from her eyes. There may also be effects on the user's personality; Kaza feels drawn to become smoke with the objects she transforms, and begins to lose normal human emotional responses. She becomes smoke in places, to the point where she has a hole in her cheek that leaks smoke. Her hair and ears have also started to transform, with single ribbons of smoke coming off each finger. This process is different from simply having transformed a portion of oneself accidentally.
Origin of Soulcasting Fabrials
- House Davar's broken Soulcaster
- Highmarshall Azure's captured Soulcaster (likely controlled by the Fused now)
- Liaforan royal Soulcaster (used by Kaza, a runaway from the royal family)
- Jasnah's Soulcaster (fake)
- House Kholin maintains a monopoly on all Soulcasters in their kingdom.
- The Azish have a few Soulcasters, including one famous for its ability to make bronze.
- The Diagram has access to at least one Soulcaster able to make things into smoke, as evidenced by the man who gave Szeth his first orders from the Diagram.
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