Tartarus is a desolate System controlled by the Drago-Kazov and at one time used as a Prison Camp for captured Warships.
When ships were captured during the Nietzschean Rebellion they were taken to Tartarus where they held pending erasure. Once reprogrammed they were to be used against the Commonwealth. Unfortunately for the Nietzscheans the AI's proved much harder to erase than expected, even when it seemed like they had been successful the original AI would always reassert itself eventually, making the crew pay for their actions. With no success it was decided to corral the Fleet back in Tartarus while they continued working on a way to destroy the original AIs. They tried invasive programs, viruses and anything else they could think of to try and break the link, even going so far as to dissect ships.
Despite the relative importance of the ships being held the System itself is unguarded with the exception of occasional patrols. The Nietzschean's confidence comes from the System itself. The Star is a Brown Dwarf Remnant of a Supernova which engulfed everything in the System when it went Nova creating a desert in space, the perfect prison for ships which needed raw materials to make munitions or parts. The Slip Route into the System is also extremely hazardous, claiming more than a few careless pilots who entered the System and were engulfed by the Star.
One ship managed to escape from this prison, the Clarion's Call a Victory's Crucible Class Drop Ship Carrier. Originally it was believed that the ship followed a Nietzschean Patrol when it left the System. Ryan, the Ship's AI wandered Slipstream for 100 years unable to Navigate without a Pilot, his only thought to find someone who could liberate his friends at Tartarus. Unfortunately there was no one left to liberate the ships and he was eventually found by the Nightsiders who turned him into a Casino where he stayed until being 'rescued' by Dylan Hunt.
The Truth about his escape was much less noble. The captured ships, 54 in all including Ryan scrounged enough to cobble together a single Slipstream Drive and selected him to go and find help. The AIs knew that the Commonwealth had lost and they also knew that Nietzschean Alliance had collapsed. The Plan was for Ryan to make an agreement with one of the Drago-Kazov's enemies. In return for their freedom they would fight for them against the Dragons. Ryan was unable to complete his mission because of his experiences during the War and in Captivity as well as the fact that there were no moderates left.
In response to the escape the Nietzschean's tore two supplies ships apart in front of the Fleet, piece by piece.
Return of the High Guard
Nearly two years after Dylan Hunt began his mission to rebuild the Commonwealth he learned of the Drago-Kazov's success in creating a device to erase the AIs being held. He immediately set out to find & rescue Ryan and learn where the ships were being held. Despite having to use force on Ryan himself he did learn the information he needed and ordered the Maru to set course for the System. When they arrived they found it exactly as Ryan had described, an empty System with 53 High Guard Warships.
Upon entering the Wrath of Achilles, the most powerful ship there they learned the truth of Ryan's escape and despite the AI's eagerness to erase Ryan Dylan, as the only Commissioned Officer overruled their Court Martial. Dylan wanted the ships to follow him immediately out of the System before the Dragons arrived with their AI Eraser but was surprised to find that many of the ships were unwilling and in some cases even hostile to the idea of going back to serve those that had forgotten about them for 300 years.
Despite this problem Dylan ordered preparations to begin for his evacuation of the System. His progress was interrupted when an unscheduled patrol entered the System with a Troop Transport, signaling the arrival of the AI Eraser. With 3 hours to go until the ships actually arrived he had no time to escape so he instead opted to set an ambush for the arriving ships. He used the Maru to attack the Patrol ships before they could leave the System while he, Ryan and the Achilles Avatar engage in a running Firefight to stop them from using the Eraser.
The plan is partially successful when one of the Patrol Ships is destroyed by the Maru, unfortunately it takes the entire missile Payload of the ship to do it and the other ship is able to escape being they can engage it. They are also able to slow down the Boarder Party but eventually a single soldier makes it to Command to activate the device seconds before he is killed by Ryan, an act which gains him much respect from the Achilles, who is now 'dying'. Dylan recalls the Maru to the Achilles to plan his next move.
With the Achilles dying and unable to function a small debate ensues over what to do, if they leave it the Dragons will have a ship capable of extreme destruction but they are unable to bring him along with the others, the only alternative is to destroy the ship to prevent its capture. The other ships in the System turn towards the Achilles and power up their engines, ready to commit suicide to destroy the Achilles and the one they believe has caused it all Ryan. Dylan eventually makes them an offer, if they come with him they can choose where they want to go after the escape, even if it means not joining the new High Guard.
Ryan is so disturbed by the idea of destroying another Hero that he takes it upon himself to assume the role of AI for the Wrath of Achilles, something that the others are unaware of until Rommie attempts the same thing. With the ships now agreeing to join Hunt an entire Nietzschean Task Force entered the System with orders to destroy the Fleet rather than let it escape. Despite the Maru's PDLs the Task Force continued to advance and opened fire on the Achilles along with other ships in the Fleet.
In the critical moment some of the High Guard Warships broke off and ran straight at the Task Force, allowing the other ships the time they needed to escape.
This last act gave the new High Guard enough ships to form two whole Battle Groups, another major step towards the renewed Commonwealth.
In classic Greek mythology, below Heaven, Earth, and Pontus is Tartarus, or Tartaros. It is either a deep, gloomy place, a pit or abyss used as a dungeon of torment and suffering that resides within Hades or the entire underworld with Tartarus being the hellish component.