Transmogrification

Crusader-transmog — The effects of transmogrification on a Crusader

Transmogrification (sometimes abbreviated Transmog, or just Mog) is a new feature in Diablo III, introduced in the Reaper of Souls expansion pack. Provided by the Mystic artisan, it is designed to increase a hero's customization. Transmogrification gives players the ability to modify the appearance of their armor and weapons without compromising any of their power or potency.

Transmogrified items can be dyed in the same manner regular items are (and retain the dye already applied). The process only costs gold: standard price is 500, but transmogrifying an item to have a legendary item appearance costs 50,000. In addition, for weaponry, the price increases if the player wishes the appearance of a high-leveled weapon, up to 50,000 in gold. Unlike Enchanting, repeated transmogrification of the same item will not raise the price. Reverting the item back to its original appearance is always free.

Limitations

Transmogrification shares a restriction with dying items in that anything that is not seen on the character, such as jewelry, belts, and bracers, cannot have transmogrification done on, no matter the quality. In addition, if there is a Legendary or Set item appearance one wants to have, he/she must find/craft that item in-game and identify it for it to be unlocked.

The amount of types each item can use for transmogrification is limited.

Class-specific items can be transmogrified into both regular equivalents (example: Crusader shields and regular shields) and class versions, but regular items can only be transmogrified into class-specific versions by the class that can use them (for other classes, it will appear as having no transmogrification).
Weapons can be transmogrified into different types (swords to maces and axes, for example). Not all types are interchangeable.

Class-specific items that do not have a direct equivalent (Wands, Hand Crossbows, Fist Weapons) can only be transmogrified within their own type.
Appearances of items that are restricted to a specific class (such as level 70 class sets) can only be used for transmogrification by the class that can wear them: other classes won't be able to see the altered appearance if they equip this item. Therefore, it is impossible to make one's items appear like those restricted to another class.

A general rule for weapons: transmogrification requires the same animation set, to avoid visual bugs. Some parallels, however, can be quite unexpected, such as Daggers and Spears.

Cross-class sets

The following item sets can be worn by any class; making them available for transmogrification by any class.

Blackthorne's Battlegear – Any
Immortal King's Call – Barbarian
Inna's Mantra – Monk
Might of the Earth – Barbarian
Monkey King's Garb – Monk
Natalya's Vengeance – Demon Hunter
Raiment of the Jade Harvester – Witch Doctor
The Shadow's Mantle – Demon Hunter
Tal Rasha's Elements – Wizard
Thorns of the Invoker – Crusader
Vyr's Amazing Arcana – Wizard
Zunimassa's Haunt – Witch Doctor

Whimsy Legendaries

The Horadric Hamburger and Spectrum are secret legendaries, found only in Whimsyshire and Whimsydale. They were designed specifically for their unique appearance. Originally intended as a joke (as the weapons are impractical for combat), they now hold value as transmog only items.