Much of what is known about Eadric comes from the Anglo-Saxon Chronicle, whose editors do not seem to have liked him. However, he does seem to have had an unfortunate talent for choosing the winning side in conflicts, usually while the conflict was going on.
Born of ignoble stock, he made himself useful to King Ethelred II. Allegedly he had a hand in the killing of Gunnhild, sister to Swein Forkbeard the [Dane]], in 1002, something history has laid at Ethelred's door, and later, at a time when four of the king's closest councillors had all suffered mysterious deaths), Eadric was made ealdorman of the Mercians, in 1007, and given Ethelred's daughter, Eadgyth, in marriage. (One of those four, Aelfhelm of Northumbria, had a daughter, Aelfgigu. At some point she married, to the son of Swein Forkbeard).
The next year, 1008CE, when Ethelred tried to raise a fleet to defend against Viking attacks, Eadric is said to have advised against this, and then to have raised feuds amongst his family which distracted the king. The implication drawn by the Chroniclers was that, in this, Eadric was complicit with the raiders. The fleet (possibly storm damaged) failed to stop an invasion in 1009, and Ethelred only got rid of the raiders by paying tribute of around �48,000.
In 1011, Eadric appears to have been among the leaders in a move to ransom Aelfheah, Archbishop of Canterbury, who had been captured by the Vikings, and tribute was paid, despite the Archbishop's insistence that no money be handed over.
During the invasion of Swein Forkbeard, in 1013CE (which resulted in Ethelred's temporary abandonment of the throne, Eadric appears to have been able to avoid the Chronicle-writers' eyes, and it is not clear whether he supported king or invader.
In 1015, Swein's son, Cnut sailed for England. Ethelred summoned a great council of nobles in Oxford. Eadric promptly murdered two of them, Siferth and Morcar, for reasons unstated. Ethelred seized their property and arrested Siferth's widow, Ealdgyth. His son Edmund Ironside, for reasons he saw good, promptly took the lady out of custody, married her, rode to Siferth's lands and, with the consent of the people there, took over Siferth's demesne.
Cnut landed, and Eadric, with 40 ships, deserted the king, and went over to him. As Cnut turned south, towards London, after ravaging Northumbria and killing its earl, Ethelred died and Edmund his son was chosen as king. Cnut besieged London, but Edmund broke out, harried Cnut's forces, forcing him to break the siege, and caught him at Otford in Kent, doing bloody slaughter. At this point Eadric swapped sides again and joined Edmund. Who, for reasons unguessable, accepted him, with the foreseeable consequence.
In battle at Ashingdon, Eadric went back over to Cnut, and Edmund was defeated, fleeing to Gloucestershire. The two agreed to split the kingdom, which lasted until Edmund died (or was murdered, possibly by Eadric) in November 1016.
Left as sole king, Cnut gave Eadric the earldom of Mercia. Then he had second thoughts (perhaps remembering his wife Aelfgifu's father, and the suspicion that Eadric might have had a hand in his death), and (according to the Chroniclers) asked Eadric how he could be sure, given his past history of betrayals, that he would remain loyal this time. To ensure his loyalty Cnut had Eadric executed. Eadgyth Cnut gave to another earl, Thorkell of East Anglia, even while he himself was marrying Ethelred's widow, Aelgifu (or Emma, sister to Richard duke of Normandy.
A play Edmond Ironside, written in the Elizabethan style, portrays Eadric as a villain of Ricardian hue, making him hate Edmund for reminding Eadric of his base birth, whilst Cnut valued him heedless of his origins. It also features an incident in which Eadric produces what he says is Edmund's head in order to induce his troops to surrender. The play has sometimes been ascribed to Shakespeare, but the attribution is widely challenged, not least on the grounds that the play is rambling and poorly organised. Eadric as a character is, however, acknowledged as a prototypical Elizabethan villain.