Neptune is one of the outer planets of the solar system. It is connected to the Shade Realm of Mind.
Like Uranus, there is very little mage activity so far from the Horizon. Several chantries exist on Neptune's various moons, but it is not clear which ones; some of them may be on moons yet unnamed by Sleeper science. The Technocracy also has a refueling station for their long-range voidships, but that's about it.