This is a time consuming quest. it requires the correct mobs to load and many svart timers.
Due to it being the first time I completed it and over the stretch of 2 months some of it might be a little foggy, but a good start
you need to find Njord and ask him Loki, he will tell you about a story of the 2 and then once you get to Loki ask him help, after unlocking him.
he will tell you he needs a favor. he will require 3 items. a swan feather, a feather of Vedrfolnir, and Nidhogg bark. the Vedrfolnir feather is timed, and not long.
the swan feather is obviously from svart, and the bark. the Vedrfolnir feather is in Asgard behind the bush. To get behind the bush find the Silhouette and ask him "Thorns". he will have the key in his corpse.
now the trick here is after you go to svart and get these items to return to loki, he tells you about him and Hel hiding an item. so you have to go into Nilf and talk to Hel. I forget what to ask loki will tell you i think Favors of the flesh. After asking Hel, she will give you the Death key. This will allow you to unlock the death chest in Svart with the trolls. So you have to wait another timer for svart, and you can't rent with the death key, witch makes it this much better.
After reaching the death chest in svart unlock it and get Thjazi's battle horn from it.
return that to skaldi in asgard for xp and whois.