The warrior bishop was an idea that interested creator Michael Hirst a great deal, and he saw Heahmund as a great foil for Ivar, the two being such wild cards. His role in the show is largely enhanced from the accounts in the history books. Because Jonathan Rhys Meyers has such an explosive performance in this role, it is likely that he has been given more opportunity to shine than the real Heahmund would have had in the history books.
In 871 the Viking army crossed the frontier of Wessex and occupied Reading. This was a royal residence and so was a collecting point for taxes and the royal feorm (food rents). As such it offered an attractive proposition to the raiders. The Viking army was led by two kings: Bagsecg and Ivar the Boneless’ brother, Halfdan. While two Viking jarls (high-ranking nobles) took a force further into Wessex to forage, the remaining invaders stayed at Reading and, according to Asser, fortified their camp by building an earth rampart between the rivers Thames and Kennet.
The West Saxons reacted swiftly to the occupation of Reading. Æthelwulf, ealdorman of Berkshire mustered the fyrd and attacked the foragers at Englefield (Berkshire), west of Reading and defeated them, killing a jarl named Sidroc. Four days later the ealdorman was joined by the West Saxon king, Æthelred, and his brother, Alfred. With their combined force they attacked the main Viking camp at Reading. In a ferocious battle the Vikings eventually gained the upper hand and the West Saxons retreated, carrying with them the body of Ealdorman Æthelwulf. It was a sharp reversal of the previous West Saxon success.
Within four days they were fighting yet another major battle. This time it was further west at Ashdown, on the Berkshire Downs. The exact location is difficult to ascertain but was probably overlooking the Vale of White Horse and on the line of the Icknield Way, a major routeway into central Wessex from the north-east. It seems that the Vikings reached the battlefield first, since Asser records that they held the high ground. The Chronicle explains that they assembled in two formations: one commanded by their two kings, Bagsecg and Halfdan; the other led by the jarls. Without giving much detail of the battle it goes on to say that King Æthelred fought against the Viking kings’ troops, killing Bagsecg, while Alfred’s troops faced the jarls and killed five of them. Both Viking armies fled before the victorious West Saxons. Asser – probably working from material provided by Alfred himself – adds the detail that Alfred began the battle first, since Æthelred had not yet finished attending Mass. The battle raged around a solitary thorn tree which Asser claimed to have seen. In a memorable phrase, Asser describes Alfred as charging the enemy `like a wild boar’.
Despite this resounding victory, and within two weeks of it, Æthelred and Alfred again faced the Viking army at Basing (Hampshire), but this time the Vikings won and the West Saxons were forced to withdraw. After this the pressure eased a little, but only two months later another major battle was fought at Meretun (the site is unidentified but was probably in Hampshire). There were a huge number of casualties and, once again, the Vikings emerged victorious. Amongst the West Saxon dead was Bishop Heahmund of Sherborne,  with Æthelweard’s Chronicle adding that he was buried at Keynsham; situated on the north Somerset border the location may have been chosen as a spiritual marker on the frontier of Wessex. As if these were not troubles enough, the Chronicle informs us that a new Viking force, the `micel sumorlida’ (great summer fleet) came up the River Thames to Reading, where they reinforced Halfdan. This may well have been the first appearance of the three Viking `kings’ Guthrum, Oscetel and Anwend, who are named in the Chronicle in its later entry for 875. Given reductions in the size of the micel hæden here due to casualties and the necessary forces required to hold down York and East Anglia, these additional forces must have been very welcome for the Vikings; and the last thing the West Saxons wished to see arriving. King Æthelred may have been seriously wounded at the battle of Meretun since, soon after Easter, he died and was buried at Wimborne (Dorset). By an arrangement that had been made between the royal brothers of the House of Wessex the throne did not pass to one of Æthelred’s young sons. Instead, it passed to Alfred. Wessex was in too great a danger for entering into minority rule and the potential instability that would have accompanied this. This shrewd piece of practical politics may well have been the major factor which saved the kingdom.
Within a month of his succession, Alfred faced a large Viking army at Wilton (Wiltshire) and lost. Asser says that an initial West Saxon advance at the expense of the Vikings was eventually reversed when the Vikings regrouped and turned on their pursuers. While the sources vary as to the exact number, it seems that perhaps nine major battles took place in 871. However, this does not take account of the many skirmishes against smaller groups of Vikings, foraging away from the main army, fought by groups led at various times by Alfred, his ealdormen and king’s thegns. By the end of the year the Vikings made peace with the West Saxons and withdrew.
 Anglo-Saxon bishops and abbots led royal armies in 825 and 848, and bishop Heahmund was killed at Meretun in 871. Warrior-clerics were not unheard of in Anglo-Saxon England, a fact that is confirmed by the celebrated military actions of notable clerics in both 1016 and 1066.
While the relatively peaceful nature of English society (or, at least, avoidance of internecine warfare) probably lessened the importance of personal military ability for English clerics, they were still expected to contribute to the defense of the realm, both through their landholding and their personal stature in the kingdom. While some contemporary observers, such as Archbishop Hincmar of Rheims, claimed that English bishops did not have the same military responsibilities as their continental counterparts, due to a lack of landed endowments, a glance through the Anglo-Saxon Chronicle demonstrates this to be false. The contents, including military hardware, of surviving wills of prelates demonstrate this, as do the attempts by reformers to prevent clerics from engaging in warfare. The simple possession of such items does not, of course, represent evidence of direct military action, nor even an endorsement of such violence by clerics, but it arguably represents a familiarity with warfare and a recognition of the role played by clerics in support of royal campaigns. The earliest of the wills comes from Bishop Theodred of London, and dates from between 942 and 951. He granted to his lord, among other things, `four horses, the best that I have, and two swords, the best that I have, and four shields and four spears.’ The inclusion of the phrase `the best that I have’ indicates that Bishop Theodred not only possessed more swords, horses, etc., than he was leaving to his lord, but that he was also cognizant of their relative value and qualities. Tis theme is reinforced by the terms of Bishop Arfwold of Crediton’s will. Bishop Arfwold left an immense amount of military gear and equipment to a variety of people, including to fellow clerics. His will read, in part, `And he grants to his lord four horses, two saddled and two unsaddled, and four shields and four spears and two helmets and two coats of mail .’ The bishop also left horses and tents to several people, including Alfwold the monk. He left his kinsman Wulfgar three coats of mail, among other valuables. He also left a man named Cenwold `a helmet and coat of mail.’
The amount, variety, and value of the military equipment even elicited a comment from Dorothy Whitelock, the editor of this section of the document. She writes,
Alfwold’s will is remarkable for the amount of military equipment and the number of horses he bequeathes [sic], in addition to his heriot and a large ship. One wonders whether he was a fighting bishop. Homilists would not have needed to preach as they do against the clergy taking part in military affairs if this did not sometimes take place, and two ecclesiastics, Bishop Eadnoth of Dorchester and Abbot Wulfsige of Ramsey, were killed at Ashingdon in 1016.