The Black Acropolis is a Dyson swarm built by a vanished civilization that was destroyed 14.8 million years ago by the enigmatic Dawn Hunters. This civilisation does not seem to have spread to any other systems, and few remains of the original builders have been found.
The Acropolis itself consists of a large number of huge black biospheres arranged into an orbiting Dyson Swarm. Some are ruined or depressurised, but the remainder contain a variety of environments including a number of Earth-like regimes. These range from very cold to near the boiling point of water.
1.2 million years ago a second civilisation of Xenosophonts arrived at the Acropolis and started to colonise the biospheres. This civilisation created diverse clades of sophont and semi-sophont creatures that have become known as the Ultimates, because of their own self-identification as "That/Us (which) cannot be surpassed". The civilisation which created the Ultimates itself has left no other trace, and in fact may have arrived in a long-distance exploration probe which targeted the Acropolis from far outside the Terragen Sphere.
The Black Acropolis, approx 1700 ly from Sol, was detected in the Interplanetary Age by telescopic observation and was the closest alien Dyson Swarm to Earth. An expedition of relativistic space adapted human colonists reached the Black Acropolis system, still inhabited by several diverse clades of Ultimates, in 4001. In the following centuries it was colonised by various other races and clades. After several decades of conflict, they settled into the Gerkletoss civilisation in the 4200s.
14.8 million years ago The Black Acropolis Builders are wiped out, apparently by the enigmatic Dawn Hunters
1.2 million years ago The Ultimates are introduced into several of the biospheres by another enigmatic civilisation
4001 - A clade of heterodox dolphin colonists from Hwii arrive at the Black Acropolis habitats, fleeing the rigidity and conservatism of Okeanism.
4023 They are joined by an expedition of Space Adapted Humans. The Dolphins and SAH contact the Ultimates and start to create a synthetic culture
4220 - Elements of The SynBios - a synclade of human, dolphin, and neo-reformist Ultimate that developed in the early decades of the Glorious Communion in the Black Acropolis habitats, sets forth to colonise further star systems.
4237 - Signing of the Gerkletoss Accord
4590 - the wormhole to the Black Acropolis is closed due to the Version War.
5768 - The Gerkletoss system is re-connected to the Wormhole Nexus
6420 - The AI part of the Gerkletoss civilisation rebels against the constraints on their development, beginning fast autoevolution. Massive use of exotic matter and spacetime engineering leaves the region extremely dangerous. A number of refugees flee to nearby star-systems in Ultimate-designed generation ships
8693 - Human and dolphin refugees from the Gerkletoss rebellion/transcendence arrive at Skiiws'nnii and begin the terraformation of that world.