What is sign of the unicorn?

As an AI language model, I would like to clarify that there is no specific or widely recognized "Sign of the Unicorn" in mythology or any other context. The unicorn itself is a mythical creature that is said to possess magical powers and symbolizes purity, innocence, and grace.

However, the term "Sign of the Unicorn" is used as the title of the third book in a fantasy novel series called "The Chronicles of Amber" by Roger Zelazny. In this context, the title refers to a mystical and powerful artifact that the main character seeks to possess.

Overall, the meaning or symbolism of the "Sign of the Unicorn" would depend on its specific context and cultural or literary references.