Subclasses do not replace the main class. They expand it. Any character can acquire a subclass regardless of race, base class, or path. Progression is granted by the GM through gameplay, separately from character level.
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Subclasses are open to any race, base class, or path. There are no prerequisites.
Subclass progression is independent from character level. The GM grants levels during play.
Each character can have at most 1 active subclass. The choice is permanent once made.