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A source is a person born with innate magical abilities, or an affinity for "the Force", which are rather difficult to control once they have bloomed. Stressful situations usually work as the trigger for the abilities' appearance. Schools of sorcery, like Aretuza and the facility in Ban Ard, were created to help these children cultivate and control their powers, lest the abilities drive them mad.
Among sources, the descendants of Lara Dorren are regarded as even more special.