This section concerns content exclusive to the Warcraft RPG, and thus is not necessarily canon.
This silk robe comes in three colors, depending on alignment. It can be white (good alignment), gray (neutral) or black (evil). Many of these robes were destroyed when Dalaran fell, but there are still arcanists making them. The robe’s wearer, if an arcanist, gains several powers. Including armor bonuses and magic resistence. If an evil character dons a white robe, she immediately loses power. The reverse is true when a good character dons a black robe. If a good or evil character wears a gray robe, or a neutral character puts on a white or black robe, she loses some power. The loss of power lasts as long as the robes are worn and disappear when the robe is removed.MM&M 146
Source
This item is crafted by tailors with a skill level of 300. Since it is bind on pickup and only wearable by Mages, there's little point in learning the pattern or crafting the robe unless the tailor is also a Mage.
Human female and Gnome male wearing the Robe of the Archmage
The Robe of the Archmage granted a relatively large Spell damage bonus to casters playing prior to the release of The Burning Crusade. The mana clicky was extremely effective for mana conservation; using that, than a gem, could get the mage through several thousand mana before needing to use Evocation. However, the Robe is not without it's downfall; it has relatively low stats compared to other similar-level Mage chestpieces such as the Zandalar Illusionist's Robe or Netherwind Robes. However, the five-minute ability to restore 375 to 625 mana effectively grants the robe an additional 25 to 41 intellect as far as its effect on the wearer's max mana is concerned. This robe was easily the best until upgrading to Enigma Vestments or another higher tier robe.
Patch changes
Patch 1.4.0 (2005-05-05): No longer triggers the cooldowns of a Mage's mana gems. Mana gems are now on their own unique cooldown category.