A fog gate is a wall of grey or white fog, which seperates different areas from each other. They also appear, before you encounter Bosses.
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 slayed the demon. By doing so the fog clears. The only exeption 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).
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, all fog gates in the area reappear idependent from the fact that you cleared them or not. These fog gates are impassable, with the exception being the one leading to a boss.
To lift those walls, you have two options:
- proceed to the area's boss and defeat it (passing through the boss' fog gate sends back invaders)
- get rid of other players by…
- sending back friendly players to their own plane using a Black Separation Crystal.
- defeat the invading player. Anything that causes death to the invader is effective.
- killing yourself
Note: An impassable fog gate is also a very good indicator that a player was able to invade you without triggering a system message.
The term "fog gate" or" fog wall" has it 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".