Legendary Craftsmanship

This is a Side Quest from The North (Act V).

After killing the mushroom king for Dvalinn and Durin, they offer to upgrade a Legendary (Purple) or Epic (Blue) item for your hero. For a price.

Walkthrough

Quest Start: In the main story, kill the mushroom king in the Old Mine for Durin and Dvalinn. Then talk to them in the Master Forge Chamber.
Fetch the Mead of Valhöll in Valholl (milk it from Gudrun the Goat) and bring it to Durin.
Fetch the Mead of Poetry from the southwest corner of Hnitbjorg (get it from a little pond) and bring it to Durin.
Fetch the Blacksteel Ingot from a room in the Land's End ~ Armory (get it from a chest) and bring it to Durin.
Reward: Durin will now upgrade one Epic or Legendary (Blue or Purple) piece of equipment by adding random affixes to it (both prefix and suffix).

Usually Epic and Legendary items don't have any affixes beforehand unless you've added them through this quest. Although there are just a few exceptions such as Black Pearl ring, Crystal Totem amulet, and a couple of more.
It is possible to overwrite already existing affixes, so you must be careful when upgrading items that have already been upgraded through this quest on a previous difficulty.
Since you can complete this quest only once per each difficulty, you can upgrade a total of three times per hero.
It is possible to see the exact information on the affixes you received through this upgrade by using external tools such as TQ Vault.

Act 5 Quests

Troubles of a New Age – Festivities – Sciron – A Northern Contact – The Warrior Princess – Heart to Stomach – White Gold – The Golden Sickle – The Power of Nerthus – Wine from the Rhine – The Troubled Son – Celtic Plaid – Little Friends – The Trapped Nixie – The Kornwyf's Scythe – Scandia under Siege – Giesel – Fir Cone Liquor – The Restless King – The Magic Cauldron – The Survivor – The Rescue – Dvergar History – Squabbling Merchants – The Craftsman's Passion – Legendary Craftsmanship – The Wisest Being – The Burning Sword