Arkaine's Valor can only be found in single player, as the reward for completing the [[Arkaine's Valor (Diablo I)|Arkaine's Valor]] quest. It's extremely useful at that point, because it is virtually guaranteed to be a considerable upgrade over any armor found to that point in the game. The usefulness of this armor declines as your character moves deeper into the labyrinth, since the AC isn't very high, but it's not uncommon for low strength characters to wear it all the way through normal difficulty and beyond.