On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough for the Lost Glory mission.
Eivor goes back to Halfdan, but encounters only Moira and feels that something is clearly wrong. It's time to investigate.
Rewards: 3400 XP.
Meet Halfdan at the Royal Hall.
Return to the royal headquarters in Jorvik to meet with Halfdan. When you arrive, you will notice that the throne is empty and king is nowhere to be seen.
Talk to Moira. The herbalist will tell you that after the recent events Halfdan's heart broke and that he became a shadow of himself. Losing Faravid and the fact that his brothers are not with him, made him paranoid and depressed.
Find and speak to Halfdan
Halfdan is in the south, patrolling the Pichering area with his faithful companion, Beonton. Go to the place marked on the world map.
You will find Halfdan next to one of the huts. Jarl found out that Eivor killed his brother, Ivarr, and did not say a word about it, making him even more depressed. In addition, he lost his hammer somewhere and does not want to go back to Jorvik without it.
Find and retrieve Halfdan's hammer
Go to the Dalby Forest (the location is marked on the world map) to find Beonton, who is guarding his master's weapon. The dog is surrounded by wolves - defeat them and pick up the hammer.
Travel to Scarborough and give Halfdan his hammer back. As a thank you, jarl will tell Eivor the story of the hammer to help him understand why this weapon is so important to him. Halfdan wants to know how his brother died and you can tell him the truth or lie to him, depending on what you did in King Killer.
Halfdan will ask you for one more favor - to light a candle at the peak of the Scarborough lantern. The mission will end and you will receive 3,400 XP as a reward.