Phasing

Phasing is a game design tool that changes a particular player's perception of outdoor areas of World of Warcraft, based on the player's accomplishments or quest progress.

Behavior
The appearance of some areas of the outdoor world can drastically change from the player's perspective, based on their progress in a quest. For example, if a player has completed Return To Angrathar, they will see Alexstrasza before the Wrathgate, and the surrounding area on fire. However, players who have not completed the quest will not see those things. These are referred to as different phases.

Idiosyncrasies
Not every player is at the same stage of every quest, and being an MMO, this has potential problems. Blizzard created a workaround by hiding players in different phases from one another, usually limited to small geographic areas. Players affected by this are sometimes referred to as phased. With affected areas, players in different phases cannot see each other, even if they are in the same party.

For example, within that immediate area of the Wrathgate, players who completed the questline can't see players who haven't. However, moving just outside the Wrathgate area allows them to see one another.

History
The technology behind phasing originated as a bug fix for an Ogri'la quest in Blade's Edge Mountains. Blizzard later re-purposed it as a solution to changing the outdoor world based on quest progress.

Extensive use of phasing began in World of Warcraft: Wrath of the Lich King. The most prominent quests with phasing are Acherus: The Ebon Hold during the death knight introduction quests, Angrathar the Wrathgate, and The Battle For The Undercity. Beginning in Cataclysm, phasing could also change the world's terrain.

Usage before Wrath of the Lich King
Before Wrath of the Lich King, phasing was used in stealth and invisibility mechanics. Events such as the opening of Ahn'Qiraj and the taking over of the Isle of Quel'Danas were simply different realm databases adding game objects and NPCs for all players.

Phasing was invented as a bug fix for the climax of the Ogri'la introduction quest chain in Blade's Edge Mountains: Into the Soulgrinder. It was later used for the Blade's Edge Shattered Sun Offensive daily quests: Intercepting the Mana Cells&dagger; and Maintaining the Sunwell Portal.

In Wrath of the Lich King
In World of Warcraft: Wrath of the Lich King, Blizzard has made full use of this system. The most well-known examples are Acherus: The Ebon Hold, which looks different to the character depending on his progress through the DK questline, and the Angrathar the Wrathgate line of quests, which end with The Battle For The Undercity (Alliance) for Alliance characters and The Battle For The Undercity (Horde) for Horde characters.

To characters having completed the questline, Alexstrasza is located in front of the Wrathgate and Varimathras, who was killed along with Grand Apothecary Putress, will no longer be in Undercity. For players on certain steps of the quest, the Undercity will also be inhabited by demons. Another example is the existence of Alliance NPCs in front of the Undercity which are not visible to players in a different phase of the quest.

Phasing is also used in (not by far a complete list):


 * Howling Fjord - the spirit world quest chain.
 * Nesingwary Base Camp, Sholazar Basin - the addition of the flight path (The Spirit of Gnomeregan).
 * The Argent Vanguard and Crusaders' Pinnacle, Icecrown - The more quests completed for the faction, the stronger the offensive against the Scourge.
 * The Shadow Vault, Icecrown - You help take over the area from the Scourge, turning it into a Knights of the Ebon Blade questhub.
 * Conquest Hold, Grizzly Hills - after defeating Conqueror Krenna in the Conquest Pit, she is phased out. Gorgonna is the leader of Conquest Hold after this.
 * Several kill quests use phasing to avoid griefing and make it more realistic (for example, the quest which includes riding on the shoulder of Gymer).
 * After players complete the quest line involving the reuniting of Brann, Magni, and Muradin Bronzebeard, the latter leaves Frosthold and another Frostborn dwarf stands in his place.
 * Dun Niffelem, the Sons of Hodir city in Storm Peaks.
 * The Snowdrift Plains and Narvir's Cradle in Storm Peaks for (and after) the quest The Iron Colossus.
 * The Fleshwerks, the area where the Scourge are building abominations in Icecrown.
 * The Forgotten Shore, in Dragonblight, which begins with mobs patrolling the shore, who all disappear after The Truth Shall Set Us Free.
 * Isle of Quel'Danas during the Quel'Delar quest chain. The entire zone is purged of hostile units, reclaimed by the Blood Elves and with a number of new NPCs on the scene. This example is unique in that the phasing only occurs for a short period of time, and following the Quel'Delar quests reverts back to its role in The Burning Crusade.

In Cataclysm
Prior to Cataclysm, phasing was unable to change the base terrain of the world. This means that the normal ground could not change in texture or positioning. An example would be the Court of Bones at the Wrathgate having burnt ground prior to being burned by the Red Dragonflight.

Cataclysm changed those limits. Terrain became able to phase, land could sink beneath the waves as well as rise above them. A raging fire was able to expand further along as time moves forward. Whole mountains crumbled and fall leading way to new lands. With this technology, you could see, for example, a harbor built before your eyes, without the use of in-game patches.

Phasing plays a huge role in Cataclysm, and not just in the newly made zones, but a good amount of the revamped zones as well. A few of the known phase changes are that of the starting areas for the newly announced races of goblins and worgen, in much of the same way the Death Knight starting zone was handled, except with the new terrain phasing feature.

Comparison to similar tools
Sharding is distinct from phasing. Sharding is used solely to alleviate player overcrowding, unrelated to the story or quests. But similar to phasing, players in different shards cannot see each other.

Layering is the same as sharding, but affects an entire realm, covering all zones, unrelated to the story or quests.

Cross-realm zones are designed to prevent under-crowding. If a zone on a realm has very few players, the game automatically merges players across multiple realms into the same "instance" of the zone. It is also unrelated to the story or quests.