On 6 of December 1344 he was proclaimed as Hereditary King. 17 of October 1356 he took actions against his father and proclaimed himself King of Sweden. 28 of April 1357 he was recognized as King over most of Sweden and Scania. His father had to agree to sharing his power with his son and later Erik's power increased even more.
Rumours said that Erik, his wife Beatrix and their infant son were poisoned by Erik's parents, but it is more likely that they died of plague.