From Diablo Wiki
You first encounter her on the quest Souls of the Dead, in Gideon's Row of Act V after you have killed Drygha. She can be found hiding under a pile of corpses in the same area. She will accompany you for a short while (not fighting) and then will return to the Survivors' Enclave to join your Caravan.
Along with delivering lore and story via her conversations, the Mystic will provide item enchanting, giving players the opportunity to reroll one affix on an item to upgrade the quality of that item. Enchanted items will become Bind on Account (BoA), making them untradeable. She is also the NPC used for transmogrification, changing your character's appearance for purely visual reasons.
 The Mystic and Enchanting
The primary service the Mystic provides is enchanting which enables players to reroll a single property (also referred to as an affix) on an item. The process is as follows.
- Take an item to the Mystic and select one of its properties you would like to change.
- Click that property and a list will appear of the new properties available to replace it with. Select which property you would like.
- Give the Mystic the necessary crafting materials and gold.
- If you are not happy with the result you can refuse the change (using up the crafting materials) and try again.
- You can only select one property from the item to replace and that is the only property you can change forever. All other properties are 'locked in' the moment you change a property.
- Any items you enchant become account bound (BoA) and therefore can not be traded or sold to a merchant.
- More on Enchanting
 The Mystic and Transmogrificationtransmogrification feature added in Reaper of Souls, which gives players the ability to change their appearance. Rather than displaying purely as their current equipment shows, players are able to change the appearance of items, (including Legendary and Set), to that of another that they have 'unlocked'. The process is as follows.
- Select Transmogrify from the Mystics menu.
- Place the item you wish to alter in the transmog window
- A list of items available will appear
- Select which look you would like and click on it to preview it on your character
- When your satisfied, hit 'Transmogrify' and for a gold fee the item will change.
- Items transmogrified will not become account bound (BoA).
- Items you unlock in Softcore mode will not be unlocked in Hardcore and vice versa.
- Items you unlock will be available to all characters on the account in that mode.
- Dye you have placed on any item will be overwritten when transmogrifying. You can redye the item after it's been transmogrified.
- Legacy items (items acquired before the expansion/pre-expansion patch) will not unlock the new looks but they can be transmogrified.
- More on Transmogrify
 The Mystic's Wagon
Like the other Artisans, the Mystic can be leveled up ten times, and her wagon changes graphics with each level. Below you can see the area she will occupy in the Hidden Camp in Act II, before she joins your Caravan and the 10 levels of upgrades she goes through as you train her.
 The Mystic's Story
Myriam is a member of the Vecin, a nomadic clan blessed with magical visions they believe come from the powerful deity known as the Allmother. Although Myriam is a particularly gifted Vecin she proves no match for Malthael's forces and they manage to trap her in Westmarch. This is where the player is to rescue her from in Reaper of Souls after which she will be at your service and join your travelling Caravan.
When the Mystic was first revealed with the other Artisans, many fans speculated that she might be Adria, the NPC witch from Diablo I. While an interesting supposition, this ultimately proved to be untrue.
 Development of the Mystic
 2010 - She's In
The Mystic was originally planned for Diablo 3 classic, and she was introduced along with the Blacksmith and Jeweler when the Artisan NPCs were revealed at Gamescom 2010. More details were added later that year at Blizzcon 2010, and the Mystic was initially set to craft spell-casting weapons like wands and orbs, along with her item identifying and enchanting duties as detailed in the Artisan FAQ from Gamescom 2010.
All of the Artisans had more varied tasks when they were first revealed. Initially the Mystic was intended to be the NPC who would craft magical items such as wands and sources, but over time all item crafting was allocated to the Blacksmith. Other planned Mystic functions were eliminated by turning them into inherent abilities (such as selling scrolls or identifying items), or removing them from the game entirely (skill runes as a type of item). With these changes the Mystic soon had nothing left but item enchanting, and the developers didn't feel that system was adding enough to the game at that point to retain it.
The Mystic's main utility comes from her ability to add bonuses to all sorts of items. This system was further detailed in the BlizzCon 2011 Panel: Gameplay & Auction House, and discussed in Blizzcon 2011 interviews. Here's Andrew Chambers speaking on the topic. 
Andrew Chambers: You specify the item you want, then you choose the enhancement you want, and that enhancement actually has a range. So say the plus experience might be 10 to 20 percent: when she applies it you might get +15% experience. You can reapply that and you’ll have a change of getting +16% or +17% experience, but it will never go beneath the current enhancement and you've always got a chance to improve to the maximum possible value.
The Mystic was never enabled in the Diablo 3 beta, but all of her in-town artwork assets were there and players were able to visit and speak with her playing a datamined version of the game.
Concept art for her wagon was shown in the Crafting Sanctuary panel at Blizzcon 2010. The basic concept was of a wagon and accoutrements with a rounded, organic shape, to make it stand apart from the Blacksmith's wagon, which as a very rectangular shape, outlined with rigid, gleaming metal and ornamented with dozens of weapons and shining pieces of armor.
A series of images showing four levels of the wagon upgrade were debuted during a panel at BlizzCon 2011. You can see a compilation of those shots below.
 2012 - She's Out
The functions of all the Artisans were reworked during 2011, and on 19 January, 2012 Blizzard announced that the Mystic would not be found in the initial release of Diablo 3. Her function at that point was deemed largely redundant to the bonuses provided by gem socketing and the team wanted to save her for later, when they could adapt her functions to fill some future roll.
The design team is currently looking at systems and cleaning them up, removing any superfluous system objectives and those that are beyond fixing. Thus, we're removing the Mystic artisan. As we look at the big picture, the Mystic simply wasn’t adding anything to our customization system. Enhancement was really just the socket and gem system with a different name, and it would prolong the release of the game even further to go back to the drawing board and differentiate it, so we’ll revisit the Mystic and enhancements at a later time. Removing her from the game took some time, but it’s nowhere near the efforts that would be required to flesh out a better customization system. We hope she’ll be able to join your caravan in the future, but for now we’re going to focus on the extensive customization options the game already offers.
When asked what happens to her enchants Blizzard confirmed:
 2013 - She's In Again
Blizzard spoke often about the Mystic even before Reaper of Souls (and her return) had been officially announced, always saying that she would be able to enchant items in some way, probably by rerolling an unwanted affix. Details about how that would function weren't revealed until Community Manager Lylirra provided some of them in September 2013.
The gold and material costs were not spelled out, as they were still under development.
- You can only reroll one affix per item, so that item-hunting remains important and you can’t slowly turn one item into a perfect item.
- You can reroll that affix as many times as you wish, even as it changes.
- When you reroll you are presented with several randomly-selected affixes and you pick one of them, which then rolls the actual numerical value and applies it to the item.
- You can always stick with the current property, if none of the randomly-selected replacement affixes are upgrades.
- They’re still debating how enchanting Legendary items will work, and if you’ll be able to reroll the inherent affixes or just the random ones.
Value: It’s important for the actual hunt for the loot to still matter How enchanting supports this: As currently designed, you are only allowed to reroll ONE property on an item. In this way, each item can only be changed so much from its original state. This allows enchanting to feel meaningful, while still allowing lots of room for players to resume hunting for an even better “base item” to enchant into an even better item.
Value: We want this to be a meaningful end-game sink
How enchanting supports this: 1. As previously mentioned, all items that are enchanted will be bound to account
2. Even though you are allowed to reroll only ONE property, you can reroll that property over and over. For example, suppose an item has 200 str, 200 vit, 80 res all, +1200 Life on Kill and +400 Life on Hit. You might choose to reroll the +1200 Life on Kill and get +87 Hitpoints per second. You can then reroll the +87 Hitpoints per second and get +4 gold pickup radius. You can reroll the “mutable” stat as many times as you want, but the other 4 stats (200 str, 200 vit, 80 res all and +400 Life on Hit) are now “locked in” and cannot be rerolled.
3. We want to ensure enchanting is meaningful on Legendary items. We are working hard to make sure Legendary items are awesome. If we want enchanting to be a meaningful end-game sink, it needs to work on the best items in the game. It has not yet been determined whether this means you can enchant the “guaranteed” properties of a Legendary, or if you can only reroll the “random” properties of a Legendary. This aspect of the design is still under iteration but at the end of the day we want enchanting Legendary items to be worthwhile.
Value: We want to avoid “Buyer’s Remorse”
How enchanting supports this: Have you ever played an RPG with an enchanting system that sometimes randomly made the item worse? Since we’re allowing the result to be random, it’s possible that your choices for enchanting are worse than the property you’re replacing. Oftentimes this can lead to a feeling of dread when you begin the enchanting process. Or maybe you re-enchant the item over and over hoping to at least get something better than what you started but you get unlucky and run out of money or reagents before you get a good roll! To mitigate this, when we present you the list of options one of the options is ALWAYS the original property. So if you don’t like the new options you can always choose to keep the old one. Now you can enchant with peace-of-mind.
 Mystic in the Beta
The Mystic is involved in the multi part quest, The Devil's Hand which involves the player having to find multiple items. Details are still sketchy as of October 2013 but some dialogue from the Mystic has been datamined and should be considered a SPOILER.
The first images of the Mystic and her shop were revealed at Blizzcon 2010. She was never enabled in the game, but players were able to see her in the Diablo 3 beta build via datamining. Her dialogues were enabled then and some players even took video of her dialogues, a pair of which can be seen below.
|The Mystic during Diablo 3 Classic beta|
Images from before she was removed from the game in 2012, preserved for posterity.
- ↑ Transmog not BoA - Blizzard Entertainment, 01/11/13
- ↑ Legacy Items - Blizzard Entertainment, 24/10/13
- ↑ Andrew Chambers Interview @ Blizzcon 2011 - Gameplanet.co.nz, 26/10/11
- ↑ Diablo 3 Progress Report - Blizzard, 19/1/2012
- ↑ The Fate of the Mystic's Enchants - Blizzard, 28/1/2012
- ↑ 6.0 6.1 Rolling Affix Method - CM, Blizzard Entertainment, 05/09/13
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