The "Prince of Sunhold" was a title traditionally used by the heir apparent to the ruling monarch of the the Borderlands. In the rare case of a female heir, the title would've been "Princess of Sunhold". The title was created by Steffon Blackgard when he overthrew his uncle and proclaimed himself the Black King of Ebonheart.
The title is currently held by Rendal Blackgard as "Lord" of Sunhold. The title is claimed by both House Blackgard and House Blackgard of Volantis. Legally, however, the title is officially the Lord of Sunhold, while Emperor Norokin desires to return the title to its monarchic roots.