Sisillius II was a mythical king of the Britons as accounted by Geoffrey of Monmouth

Geoffrey of Monmouth

Geoffrey of Monmouth was a cleric and one of the major figures in the development of British historiography and the popularity of tales of King Arthur...

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Mythical account

Sillius II was the son of King Guithelin

Guithelin

Guithelin was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He became king after the death of Gurguit Barbtruc.He ruled liberally and temperately for his life. His Queen consort was an artisan and noblewoman named Marcia. When he died, his wife took over the government...

 and Queen Marcia

Queen Marcia

Queen Marcia was the mythical third queen regnant and a regent of the Britons, as recounted by Geoffrey of Monmouth.-Mythical Account:Marcia became queen after the death of Guithelin and ruled as regent for her son, Sisillius II. In her youth, she was the Queen consort of Guithelin, the king of...

, succeeded by his son Kinarius

Kinarius

Kinarius was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He was son of Sisillius II and succeeded by his brother, Danius....

. Since his father, Guithelin, died when Sisillius was just seven years of age, his mother, Queen Marcia ruled Britain for about five years in his stead. Upon her death, in ca 358 BC, Sisillius came to the throne, ruling for the next six years.

His reign was followed by those of his sons Kinarius and Danius.