A fog gate is a wall of grey or white fog, which separates different areas from each other. They also appear before you encounter Bosses.
General Role
When you get close enough to a fog gate, you can choose to traverse the white light and enter a new area or a new room. In this case, the fog clears and you don't have to traverse the fog again, to move from one area to another.
But be careful, if you went through a fog gate, which is leading to the confrontation with a boss-demon, it won't clear and is unpassable until you slay the demon. By doing so the fog clears. The only exception is the fog wall which you have to traverse before encountering Seath the Scaleless the first time. During that fight, you can leave the room at any time, because you can't win the fight (losing the fight is necessary in order to progress through the game).
You can also warp out of a boss room using a homeward bone item, the homeward miracle, or by using the dark sign. Beware though that those only work when there are no phantoms present. If you do have phantoms with you, you can send them home with your black separation crystal
There are several orange fog gates in the game which can only be lifted by obtaining and placing the Lordvessel at the Altar.
These fog gates can be found in Anor Londo, Tomb of the Giants and Demon Ruins.
Role during Multiplayer
When you summon other players to assist you or if you are forcibly invaded in any way, fog gates appear to delineate the area, often preventing the player from accessing bonfires or leaving the area.
Invasion / Coop related fog gates cannot be traversed either by the player or the phantoms and whilst the player can still traverse boss room fog gates, invading phantoms cannot, and cooperative phantoms can only after the player does.
Lifting fog gates
Mid-level fog gates
You can traverse these at any time by pressing the appropriate button. As soon as you traverse them they will dissolve and not reappear even upon death. They only reappear on NG+
Boss-room fog gates
You can also traverse these at any time. But they are one-way with the only exception being Seath the Scaleless' initial encounter. They can only be lifted by defeating the boss.
Coop/invasion related fog gates
These cannot be traversed by any entity in the game. Neither player, phantom, or enemy. They serve to confine the battle arena to a limited area. These fog gates will automatically dissolve when:
- The player proceeds through the boss fog gate. (Doing so will send any invading phantoms home and unlock the fog gates for cooperative phantoms)
- The player kills all invaders present and sends home all friendly phantoms.
- The player dies
Note: often the first signs of an imminent invasion are the appearance of fog gates and the greying out of your homeward bone/homeward miracle.
Trivia
The term "fog gate" or" fog wall" has its origins in Demon Souls. There, the world was hulled in some kind of evil fog. So during gameplay, you had to go through that fog to proceed. In Dark Souls, those gates are called "the white light".
