Phasing is a technique commonly used in MMORPGs. This refers to the technique of having a certain area look differently to different characters.
How it Works
Data on the location of game objects, players, and NPCs is not stored in the World of Warcraft client and must be sent from the server. Phasing, in a sense, decides not to send certain data based on criteria.
As an example, you may have a quest to get the head of a named NPC. After turning in the quest, the server records that it should not send data about that NPC to you. The next time you go there, despite if the NPC is still there, you will not be able to see it.
By using combinations of phasing, entire areas can change from grasslands to a city by use of game object phasing and NPC phasing.
Appearance
NPC phasing is recognized by a NPC which should be at a location, but is completely gone when viewed by someone else.
Player phasing is recognized by not being able to see a player. Early uses of this was stealth and invisibility effects which caused the server to only show hidden players to certain people.
Game object phasing is the easiest to recognize and usually requires multiple phasing techniques. Game objects can be anything from a chair or chest, to a building. In most cases, players that can interact with phased buildings must also be phased, otherwise it would appear that players were walking in the air.
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.
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 differently to the character depending on his progress through the DK questline, and the Angrathar the Wrath Gate line of quests, which end with Quest:The Battle For The Undercity (Alliance) for Alliance characters and Quest:The Battle For The Undercity (Horde) for Horde characters.
To characters having completed the questline, Alexstrasza is located in front of the Wrath Gate 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):
- 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 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 Quest:The Truth Shall Set Us Free.