Summary
The Dark Fog Space Hive is the center of the Dark Fog's operations within a star system. It gathers energy from the system's star and distributes the energy to Relay Stations, which in turn provide it to their corresponding Planetary Bases. In exchange, the Relay Stations send matter back to the Hive, which it uses to build structures and vessels that may serve defensive or manufacturing purposes.
Mechanics
The Hive and its Relays have a mutual relationship, in which each of them assists the other with resources. They also cover for each other in the event that the other is destroyed: the Hive can manufacture new Relays to deploy, and established Relays can send matter to reconstruct a destroyed Hive. In order to truly wipe the Dark Fog from a system, the player must deal with both the Hive and its Relays.
Normally, the Hive's threat level does not increase at all. However, if Icarus goes on the offensive and attacks a Planetary Base, the Hive's threat level will increase by a small amount. Additionally, attacking an established Relay will dramatically increase the Hive's threat level. Once the Hive launches an assault, it sends space units to attack the planet that Icarus was on when the assault began.
Once a Hive has grown large and accumulated enough matter and energy, it can produce a Seed, which flies to another star system to establish a new Hive. Seeds travel fairly slowly and are completely defenseless, so the real danger of Seeds comes from failing to notice them. Once the Seed takes root, the new Hive is still weak and easily destroyed, but it is much harder to spot on the star map and can quickly grow while flying under the radar. Alternatively, if the Seed arrives in a system that has a destroyed Hive that has not yet been rebuilt for some reason, the Seed will rebuild that Hive instead of starting a new one.
Combat Capabilities
The Space Hive can manufacture two types of space fighters: Humpbacks and Lancers. Humpbacks are the first units to be produced early in the Hive's growth. They patrol around the Hive and will attack assailants, but they will not leave the Hive to launch an assault or to pursue retreating attackers. Lancers are mass-produced once a certain amount of Humpbacks are active, and they form the majority of the Dark Fog's space force. At maximum, a single Hive may have a defensive perimeter of Lancers, up to 12 additional fleets of Lancers, and a fleet of Humpbacks protecting its core.
Space Hives are far from defenseless even without fighters, however. A sufficiently developed Hive has many High-Energy Plasma Sentries installed periodically on its structural frames. These deal enormous amounts of damage, especially when several of them fire on one target. Even if Icarus is heavily upgraded with late-game Technologies, these sentries can shred through it quickly if it gets close.