Basim betrayed Eivor in Assassin's Creed Valhalla as he is the reincarnation of Isu Loki and wants justice for the unfair treatment that his son received. Basim wants Eivor and Sigurd dead as they are reincarnations of Isu Havi (Odin) and Isu Tyr, who treated Loki's son, the wolf, unfairly.

Why does Basim want revenge?

His campfire story wasn't from the reality of the 9th century, it was from the eons before when they were Isu. It transpires that Basim is the reincarnation of Loki in Eivor's Norse Isu past, meaning that he wants his fellow Isu dead in revenge for their treatment of his son.

Is Basim good or evil?

Eventually, though, Basim was revealed to be the bad guy all along who wanted revenge on Eivor for his past wrongs as Odin during the First Civilization. This evil scheme turned Basim into a hammy villain, also being too “out there” to fall in line with his characterization.