Transmogrification (commonly referred to as transmog, tmog, xmog or simply mog), implemented in Patch 4.3, provides the ability to exchange the appearance of your armor and weapons with those of other items. However you must possess and be able to equip both items, armor must be of the same armor type, and weapons must use the same attack style. Not all items or pairings can be used in the transmogrification process.
To transmogrify gear, first obtain items whose appearance you wish to impose on your equipped gear, and take them to a transmogrifier NPC:
Grand Expedition Yak had its reforger changed into a transmogrifier in 6.0 with the removal of reforging. A Transmogrifier can be added to your Garrison as well through the level 3 Storage.
Note that each of these NPCs is accompanied by the NPCs who offer the void storage and reforging services.
Right-click on the transmogrifier. You will be presented with the transmogrification interface, which shows your current equipment, similar to the standard character equipment window. Worn equipment which has not been transmogrified will have gray icons, while equipment which has been transmogrified will bear the colored icon of the items whose appearance they are displaying.
To transmogrify an equipped item, mouse over its slot to have an arrow appear. Click the arrow to view a list all suitable items in your inventory, bank, and void storage. Click on the item whose appearance should be used. You can also right-click items in your inventory or drag them onto transmogrification interface.
You will be shown your character model as it would appear after the transmogrification, plus the fee that will be required. You can continue to try out the appearance of different items in different slots. Finally, click the "Apply" button. You will be charged the fee and your equipped items' appearance will change.
The cost to transmogrify any item is its vendor value, with a minimum of 1 at level 90. Lower-level characters have an adjusted minimum cost.
If you don't want to perform the transmogrification, close the window or walk away.
The Undercity is the capital city of the Forsaken undead of the Horde. It is located in Tirisfal Glades, at the northern edge of the Eastern Kingdoms. Far beneath the ruined capital city of Lordaeron, the royal crypts have been turned into a bastion of evil and undeath. Originally intended by Prince Arthas to be the Scourge's seat of power, the budding "Undercity" was abandoned when Arthas was recalled to aid the Lich King in distant Northrend. In Arthas's absence, the Dark Lady, Sylvanas Windrunner, led the rebel Forsaken to the Undercity, and claimed it for her own. Since taking up residence, the Forsaken have worked to complete the Undercity's construction by dredging the twisted maze of catacombs, tombs, and dungeons that Arthas began.
Subject to certain restrictions, gear from nearly any source can be used to transmogrify, such as: world drops, quest rewards, badge vendors, and crafting.
For the dungeon sets 1 and 2 from Vanilla, which are no longer obtainable, you can buy replicas from vendors at the Darkmoon Faire. These items were specifically made available for transmogrification.
The auction house is a good source as well, though desirable items may command high prices.
A transmogrification set or transmog set, more broadly categorized as a vanity set, is a group of items of the same armor type designed with the same visual style and spanning at least two armor slots. A character wearing the set or a transmogrification of the set will tend to have a consistent appearance, so they are often collected purely for this purpose. They should not be confused with gameplay-affecting armor sets, which grant set bonuses when worn together.
"Transmog set" may also refer to any group of items to which a player has (or had, or intends to have) his or her gear transmogrified. This may range from a true vanity set designed by Blizzard artists, a mix of items custom selected to achieve an unusual in- or out-of-universe look, or a simple grab-bag of preferred items with little regard given to matching or composition.