At the end of King Killer, once you have defeated Ivarr, you’ll need to decide whether you’re going to send him to Valhalla by giving him his axe, or deny him Valhalla and walk away. The first choice has the least potential consequences, but you can still choose to deny Ivarr Valhalla without any negative impact.
Send Ivarr to Valhalla
Eivor gives Ivarr his axe. Ivarr says that he hears the wings of the Valkyries and dies soon after.
Deny Ivar Valhalla
Eivor condemns Ivarr to Helheim. Ivarr dies soon after, distraught.
For more on the consequences of sending Ivarr to Valhalla or denying it to him, and how this connects with what you tell Ubba later on, see Ivarr Died Well, Ivarr Died Badly, or He Wanted Death?.