In the twelfth-century Danish history Gesta Danorum (The Deeds of the Danes), Siward, (Sywardus, Synardus), was an ancient king in Götaland, who had a daughter named Alfhild, who became a legendary Viking pirate.

To protect his daughter from unworthy suitors, Siward had her chamber guarded by a lizard and a snake. He also said that if any man tried to enter it, and failed, his head was to be taken off and impaled on a stake.

When Alf, a Danish prince, managed to defeat the animal guards, Siward told him that he would only accept "that man for his daughter's husband, of whom she made a free and decided choice". At first, Alfhild said no and ran away from home, but, after some adventures with her Viking fleet, she met Alf in a sea combat, and, after losing the battle, decided to marry him.

Siward also had two sons, Wemund and Osten.

The name Synardus seems to be a medieval or modern invention.^[1] In the original Latin text of Gesta Danorum, it is rendered Siwardus and Sywardus. In Danish, it is Sivard.

• Alf and Alfhild
• Yngwin
• Awilda