The star cluster that serves as the heart of the Turian Hierarchy, and therefore arguably the most secure in Citadel space.
Unfortunately, the Reaper assaults here were brutal. What were once highly-developed colonies, scientific outposts, garrisons, fueling stations, or mining infrastructures were largely reduced to scrap by the war.
- Calum System: Anennos (Calum II).
- Castellus System: Datriux, Carborix, Nios, Digeris, Fiax, Iritum.
- Farin System: Farin II
- Gemmae System: Pheiros.
- Synde System: Armos Tevote.
- Trebia System: Aventen, Caelax, Palaven, Impera, Essensus.
- Planetoid HK-2213, streaked with veins of iridium and palladium, and possessing an abnormally large concentration of the heavy metals used in many modern alloys.
The cluster was invaded early in the war, shortly after the fall of Earth, via the Mactare Relay (See: Mactare Nebula). Staying calm, the turians massed a force around Palaven. Fleet Admiral Coronati, in what became known as the “Fifteen-Minute Plan”, stationed only two carriers, Undaunted and Resolute, near the Trebia relay. When the Reaper fleet emerged, the carriers launched swarms of unmanned fighters and spy drones. These were quickly destroyed, but the drones transmitted vital data on the Reapers’ effective range, fleet composition, and exact location. The turians’ other ships then deployed to defend the system in earnest.
Knowing that the Reapers’ weapons had a longer effective range than any of his own, Coronati made a short, daring FTL jump, landing his dreadnoughts in the middle of the Reaper fleet. The dreadnoughts then turned to line up their main guns on the Reapers, which also needed to turn if they were to fire on the turians. This ploy used the Reapers’ size against them -- because they could turn faster, the turians’ concentrated firepower downed several Reaper capital ships. The destroyers, however, then performed a jump of their own to the skies over Palaven, beginning orbital strikes of turian cities. The turians, forced to defend the planet, found themselves in a pitched battle far from the relay, from which emerged a seemingly endless line of Reaper ships. After massive casualties, Coronati ordered retreat.
As the war for the cluster ground on, troops from Invictus began to mutiny, voicing dissatisfaction with current orders and demanding their immediate release from service in the Apien Crest, and that the Hierarchy provide necessary shipping for the return of the entire expeditionary force to their homeworld. This request was echoed by their planetary government.
Over the course of the war, most arable land in the Apien Crest was destroyed by the Reaper onslaught, and what remained was used by the Reapers in attempts to cow the turian government into collapsing or capitulating. With typical turian resolve, these manoeuvres were resisted, but in the process many turian colonies were turned into apocalyptic wastelands, almost completely devoid of the means to produce food for the impoverished survivors.
Elsewhere in the galaxy, having remained free of the Reaper threat, the largely agrarian colony of Solregit became an important source of food and resources for soldiers around the front. With that responsibility, the troubles concerning the planet’s long-standing feud over separatism only grew more pronounced. Local regiments the 895th Primarch’s Own and the 58th Desert Bats travelled to the Apien Crest to join the fight for the home system, leaving a network of local garrisons to defend against potential Reaper attacks on Solregit.
The Reapers paid dearly for their attack, but so did Palaven. Almost all of the planet was in ruins, entire cities and battlefields levelled by weapons of mass destruction -- used both by the Reapers and the turians themselves. The turian martial spirit prevailed, however, complemented by the single-minded strength of the krogan, and reconstruction continues at a brisk pace; helped along by strong community drives to collect scrap and renovate ruined buildings, though time estimates on planetary recovery are pessimistic. Predictably, Palaven houses one of the largest post-war military training grounds in the galaxy, where divisions of turians, krogan and humans participate in joint training operations.
Most of the refuelling apparatuses and the antiproton distribution center at Essensus were obliterated by the Reapers, undoing decades of work developing the vast network of helium-3 skimmers and depots that sprung up around the planet. The Hierarchy hadn’t forgotten its legacy, however, and it’s been the centre of a committed initiative to rebuild the planet in its former image. Though its current output is nowhere near its former levels, it’s still the Apien Crest’s most productive fuel depot.
See also: Siatex. This post-war mining project embarked on a massive campaign of high intensity planetary and asteroid mining; deploying small fleets of ships and drones, multiple mobile industrial platforms, and thousands of sapients across eight (later nine) star systems.