Each spec gets a unique weapon--36 weapons total. These are intended to be ultimate weapons for you and are famous weapons¡ªDoomhammer, Ashbringer.
There are unique quests for each spec to reflect their flavor. Examples: Ret Paladins return to Broken Shore and fallen heroes to retrieve Ashbringer, Protection Warriors discover legend of Vry'kul King, Death Knights retrieve shards of Frostmourne from base of Icecrown. These quests are intended to be one of the first things you do in Legion.
You get your weapon at the start of Legion and it stays with you for the entire expansion.
Artifact weapons have two progression paths.
Artifact Power: gained from most activities in Legion and it is spent on traits. Eventually you can unlock all traits, and you choose the order in which you unlock them. Some traits are more powerful than others and require more power. Example - Echo of the Highlord - When you use Templar¡¯s Verdict or Divine Storm, Ashbringer will mimic that action after 2 sec.
Relics: drop from traditional sources, like dungeons, raids, quests. Relics are socketed into your weapon for DPS/stat boosts, as well as perks that enhance traits. A relic from Mythic will be a more powerful relic than one from LFR. Your loot specialization determines what type of relic you¡¯ll get. There are no weapon drops in Legion and Relics take the place of weapon drops.
There are planned catch-up mechanisms for off-spec artifact weapons, you can mostly catch up to your main spec's progress, but having multiple weapons optimized will take effort..
Players can change the design of their weapon, as well as tints.
These designs are unlocked through a variety of ways--some will be via leveling up your weapon once you reach max level, like the "Master" version of your weapon. Others are tied to achievements, killing enemies related to the Artifact Weapon, or visiting important locations.
Feral and Guardian Druids, since their weapons are hidden while shapeshifted, will have their appearance change instead.
The new PvP system will have special Artifact variations as rewards.
You can transmog your Artifact, but you cannot transmog other weapons to have its appearance.