On this page of the AC Valhalla Dawn of Ragnarok guide you can find a walkthrough for Legendary Hoard side quest. The quest involves finding treasure in Svartalfheim to which you'll be led by 4 treasure maps given to you by your companions.
After completing the Svartalfheim storyline, you'll be taken back to Ravensthorpe. A short quest awaits you there - Legendary Hoard. Start by visiting the campfire - its location is marked on your map. You can either visit it both during daylight and nighttime.
As you sit by the fire, there are 4 dialogue options to choose that summon a different character. Each will tell you a story and give you a treasure map. These lead to various resource cashes hidden in Svartalfheim.
You can view the locations of each hoard on a dedicated page of our guide.