Before the event of the game, Erik gave Mia a necklace for her birthday. This necklace appears to give Mia the power to turn any matter into gold. At first, Erik is too excited about it, however he soon find out the necklace is dangerous when his sister show him a golden seagull. So Erik told Mia to take off the necklace, however, the necklace loses control and turns Mia into gold, much to Erik's horror and regret. He than leaves his golden sister in their hideout and left.
After the fall of Yggdrasil, Mordegon turna Mia back to normal and weaponising her greed and anguish. Turning Mia into Gyldygga, who turns the Vikings that used to enslave them into golden monsters and than put a curse called Gold Fever into Sniflhiem. Which turns it citizens into gold randomly. She also built a golden castle called Gyldenhal.
When the Luminary arrived Sniflhiem, Gyldygga has her minion to kidnapp Erik. After rescuing Erik, the Luminary is able to recover Erik's memory through a Yggdrasil root. They than storms in Gyldenhal and is shocked to find that Mia has become Gyldygga. Although Erik tried to reason with her now-evil sister, Gyldygga refused to listen and attack them. In the end, Gyldygga is defeated and Mia returned to her sense, destroying the Gyldygga personality as a result.
As a human, Gyldygga is basically Mia. However, when in her monster form, she is a golden beast with sharp claws
Gyldygga is extremely greedy. She also has a hatred towards Erik for abandoning her in their hideout.