This page of the Assassin's Creed Valhalla guide has a detailed walkthrough for The Forge and the Flame quest.
Reward: 3400 XP.
Gunnar and Brigid want to get married, so it's time to throw them a wedding. Access to this quest unlocks after conquering England - after completing the Hamtunscire storyline.
Speak to Gunnar
Talk to Gunnar and ask about his wedding with Brigid. Next, tell him that it can be organized now.
You will watch an extended cutscene in which Gunnar marries Brigid. After the ceremony, you can speak with the guests who are present at the ceremony.
Speak to Randvi
Talk to Randvi and offer to keep her company. Then follow her and listen to what she has to say about your success in England and Eivor's leadership to the Raven Clan.
As you sit by the stream, Randvi will reveal that he and Siegfried are breaking up. After the conversation, the mission will end and you will receive 3 400 XP as a reward.
If you haven't started a romance with Randvi before, you can do so now, as an additional dialogue option will appear on the screen where Eivor can confess his love to her.