To start the quest, go to dwarven ruins (M3,6) in the Longwatch Falls. You will find a sword in the rock there. You can pull it out with any character that has 16 Might points or 11 Intellect and 13 Might points at the same time. It is best to select a character that uses two-handed weapon - a warrior, a barbarian or a paladin. Once you pull the sword, it will connect permanently to the person that obtained it and a clue as to where you should go now will appear on its blade.
Important! Until the quest is over, the character must use that sword. You can't switch it to any other weapon!
Second verse of the poem will tell you to search for the place where adra, stone and the copper meets. You will have to leave the White March and return to the Dyrwood. Go to one of the first locations you visited in the game - the Cilant Lis and walk towards the adra obelisk. You will be able to interact with it. Another verse will appear on the sword.
This time you will have to go to the Esternwood which can be found east from a Gilded Valley. There you must go to the cemetery located in the northwestern border of the map. Once you reach the middle of the field, simply use the camp materials and rest. After waking up the party member equipped with the sword should talk. The last verse will appear on the sword.
You must find the adra fingers. The only place where something like that can be found is your fortress - the Caed Nua. Once you get there, search for few rocks sticking from the ground. You will be able to interact with one of them. It can be found more or less in the center of the location. The quest will end successfully. Now you will be able to remove the sword from the hands of your character. However, only the person that pulled it from the rock will be able to use it. After all upgrades it will have many additional effects and can be used until the end of the game.