Once you’re pretty much done in Vanaheim, having already taken on and beaten the Nidhogg, your main quest will take you back to Sindri’s house, but there is the option to stick around and explore and do some side quests.

If you stick around you have to clear the path in the water with a Sonic arrow. Once you’ve done that and started on your way, Freya will ask for your help. This will kick off the “Freya’s Missing Peace” favour/side quest.

In it you will have to travel to Goddess Falls in the southwest of Vanaheim and assist Freya. All you need to do to retrieve Mardoll (which is actually another sword for Freya), complete the “Freya’s Missing Peace” favour - which takes place in the Vanir Shrine region.

The only potentially tricky aspect is getting access to the southeast area of Vanaheim. To do so, head to Cliffside Ruins and set fire to the bramble at the top of the cliff, which will drop the chain down from up high. Then all you need to do is climb up and interact with the blue rune surface and use the magic chisel. This will open the path so you can start and finish the side quest.