Diablo/Lachdanan

At hell level 14, find the legendary warrior named Lachdanan (he looks like a black knight). He will tell you more details about what happened at Leoric's chamber and that he is doomed to walk on the world as a demon. His mankind is almost running out and his goodness too, so he will ask you to hurry and save his soul, since he is unable to search for goodness. He will say that in order to save him, he will need to drink a golden elixir, somewhere below this level.

Go to level 15 and search for the elixir. It on the floor, probably surrounded by several demons. After getting it, backtrack to Lachdanan and handle him the elixir. He will thank you so much and drink it. As he vanishes, he will drop his helmet as a reward, the unique item: Veil of Steel.

The Veil of Steel is the most defencive helmet in game, so it's an helmet that surely you will want to use. However, it sacrifices a lot of your light radius.