This quest can begin in your longhouse at any point after completing a pledge to a territory. Sigurd will be waiting for you outside the room, and he will notify Eivor that Ubba has come to see him. Speak with Ubba, who can be found speaking with Sigurd nearby.
All you need to do to complete the quest is speak with Ubba. A cutscene will play, during which Ubba will thank you for sending Ivarr to the fate he deserved, and for being honest about it. As a token of thanks, Ubba will gift Eivor a silver ring. This can be sold at any merchant for 63 silver.