I like the rules so far! Pretty cool.
Soulcaster Fabrials are ancient items which modern artifabrians cannot replicate. They come in two types.
Limited Soulcaster – 90% of Soulcaster fabrials are limited. These items provide a Transformation rating of 4, but can only be used to perform one type of transformation (such as stone to grain). They are rare, precious, and religiously valued.
Full Soulcaster – These items provide a Transformation rating of 4 and full access to the Surge. They are rare and valuable beyond anything in the world except perhaps Shardblades and Plate.
Soulcasting Savantism. Oddly, continuous use of the power of transforming others makes one's physical form become stonelike - rigid and inflexible. A Soulcasting Savant gains one die and one Nudge on all Transformation rolls, but loses one die on Physique rolls based on agility/dexterity.
With Oathbringer, the Soulcasting Savantism and Soulcaster Fabrial rules can be polished up. Something like this, maybe:
Minor Oathbringer spoilers (do I still have to put these in OOC?):
Limited Soulcaster Fabrials: A character using a Limited Soulcaster gradually takes on the qualities of the resulting substance that the fabrial is designed for. If a character spends more than 50 charges of Stormlight in a single session to successfully Soulcast, then they start to receive Burdens, with the severity depending on the number of charges (above 50) that are spent.
1-10 charges = Serious Burden
11-30 charges = Grave Burden
31+ charges = Mortal Burden
Note that these Burdens are only applied if a character has already spent 50 charges on successful Soulcast rolls during the current session. Charges spent on failed Soulcasting attempts are not considered when applying this rule. Also, Burdens are received for individual instances of Soulcasting (so there's no need to keep track of how many Stormlight charges you've used to Soulcast in the current session beyond the first 50).
Serious and Grave Burdens obtained in this fashion can only be healed through supernatural means (Regrowth, Gold Feruchemy, insane amounts of Stormlight, etc). Mortal Burdens obtained in this fashion cannot be healed, ever.
Only Burdens are inflicted, not Damage. This means that the character does not also lose Health as a result of using a Limited Soulcaster fabrial.
The character can take advantage of one of their own Burdens to add an appropriate number of dice to any future Soulcasting rolls involving the same Limited Soulcaster fabrial that inflicted the Burden in the first place.
For the purposes of this rule, a Short Breather will reduce the count of "successful Soulcast charges spent in the current session" by half, rounded down (as long as the character doesn't Soulcast anything for the duration of the Breather). A Long Breather will reset the count back to zero (again, only if they don't Soulcast).
With Narrator approval, any character who has either the Transformation Surge power, a Full Soulcaster fabrial Prop, or a Limited Soulcaster fabrial Prop can become a Savant in one
of the Ten Essences. A character with only a Limited Soulcaster fabrial Prop can only become a Savant for the Essence that their fabrial specializes in.
A Savant gains two "free" Nudges for any roll made to Soulcast an object(s) into their chosen Essence or its Related Materials. The Savant loses two Nudges for any roll made to Soulcast an object(s) into a different Essence or its Related Materials. If this decreases the number of Nudges to below 0, then the Result of the roll is reduced by 1.
A character with the Transformation Surge power or a Full Soulcaster Prop may become a Savant for multiple Essences.