Follow the story until you complete the gem quest that brings you to fortress of the shield and after return to coparann. I had another background quest for the spell sword (takes place at this time) and required I return to coparann. Once I finished and was on my way out of the city an elf lady stops me and ask if I’ll deliver the sword to Arwin Merton and is it’s own achievement but is need.Then after you collect all the rest of the gems return to the tavern where it all began and talk to Arwin. I did it right before I return to where I found the crown.Arwin will recount a tale about a lost temple and then confirms its true and that he has been there with others but everyone else died. There dialog I got it on the first play through so there might be a roll but I believe if you start the dialogue it gives the quest. Then it’s a straight forward go to location by the lava forest and fight a legendary creature. I was lvl 11 and it wasn’t a challenge for my party. If you don’t have the right background just pop in and look around and the quest to deliver the sword should still appear just having the background makes you go back and is how I found the quest