Cultures spread across the globe have mythological centers of power underground, and many specifically part of a Hollow Earth. Agartha (Agartha on Wikipedia) is a specific version of this Hollow Earth concept, developed in the 19th Century.
The three faction versions of the quest that leads a character to Agartha are: Might Makes Right (Templars, Training Day (Illuminati), and Chaos Practice (Dragon). Once the character enters, they see the cutscene shown in the video.
The uniformed "Station Master" that explains Agartha to the player character has a mercifully and tastefully succinct number of similarities to the conductor from 2004's The Polar Express
The other branch leads to the main game areas, in order of release and their place in the main quest line:
- Solomon Island: Kingsmouth Town, The Savage Coast, Blue Mountain
- Egypt: The Scorched Desert, The City of the Sun God
- Transylvania: The Besieged Farmlands, The Shadowy Forest, The Carpathian Fangs
Possibly a Tokyo teleport was added at the end of these sections of the same branch, or another portal was added elsewhere. Please confirm.
Usually characters are only on their way to a mission when they pass through Agartha, but there are a small number of missions that take place there.
The bees that were one of the first signs of the player character's transformation into power are inextricably linked to the magic of Agartha; in one of the later Tokyo missions, Orochi scientists are seen to be attempting to graft the abilities of the bees to enter Agartha, to mechanical creatures.