In January 878, following the Battle of Chippenham, Guthrum became disillusioned with the cause he was fighting for and looked to his enemies' religion for answers. In May of that year, after being defeated at the Battle of Edington, Guthrum conceded to Alfred and converted to Christianity and taking on the name Athelstan.
In 885, Toka Sinricsdottir arrived in Ravensthorpe to convince Eivor to help aid in the siege of Paris. Unaware that Guthrum had changed his name and was comfortably ruling East Anglia as per the terms of his surrender to Alfred the Great, Toka claimed that she sought Guthrum's help, immediately arousing both Eivor and Randvi's suspicions regarding the true purpose for her visit.
Personality and characteristics
Despite being a mighty conqueror, having great strategic abilities as well as being a feared Viking leader, Guthrum had long grown disillusioned with the countless deaths he has come across in his conquests, from allies as well as enemies. Despite this, he still kept his nihilistic feelings to himself in order to remain in strong terms with his people. 
When advising Eivor, Guthrum showed his preference for using strategies which mitigate the lives lost in his conquests on both sides. This may have been inspired by his disillusionment of his conquests. He was deeply affected when his men died by Fulke's men and even expressed grief at Soma's death. 
After the Battle of Chippenham, Guthrum confessed his disillusionment to Eivor, who herself had started to grow disillusioned by the cause she was fighting for as well. He secretly confessed to her that he held great respect for Alfred the Great's inner strength which wad fueled by his strict faith in God. And this in turn was inspiring his interest in Christianity. 
Behind the scenes
- Assassin's Creed: Valhalla (first appearance)
- The Siege of Paris (mentioned only)
- Assassin's Creed Valhalla: Geirmund's Saga