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The Gate of the Worlds
The Gate of the Worlds is a name given by the unicorns to a mystical power, allowing one to traverse space and time.
Ciri is a prominent user of that power, having travelled through numerous times and dimensions (for example, she seems to appear in renaissance France and other parts of Europe during the medieval era, where she meets Teutonic Knights) who give her the title of "The Lady of the Worlds".
Alvin is also likely to have possessed it to some degree.