The Endless Swamp was located on the Wild Space planet of Zakuul, named by the natives for its sheer size, it was scarcely populated and devoid of any technology. It was home to many immigrated variants of some creatures, including the Mawvorr from Voss, the Wingmaw from Balmorra, the Lurkers, who were believed to have originally come from an unnamed planet near the Torch Nebula, and the rancors from Dathomir. The only known native species was the arachnid Iknayid.
Over a thousand years before the Galactic War, the Gravestone, said to have been the only ship to have ever gone up against the Eternal Fleet and achieved victory, was scuttled by it's owners in the eastern Endless Swamp. It would remain there for the next few centuries.
A swarm of Iknayids native to the Endless Swamp.
In 3631 BBY, The Commander, convicted of the assassination of Emperor Valkorion was rescued from Emperor Arcann's by Lana Beniko, Koth Vortena, HK-55 and T7-O1. However, their transport was shot down in the Endless Swamp. HK-55 detected a 'large metallic object', and the group, minus T7, who were in dire need of transport off Zakuul, decided to investigate. Discovering the Gravestone, which was in a surprisingly good condition, The Outlander helped Vortena scavenge starship parts and a fuel cell, and assisted Lana in finding a functioning water filter. Not finding any functioning filters, they instead discovered a fresh water spring. HK-55 located a camp of exiles, who had protested Arcann's rule, and were on the run from the Eternal Empire. The Outlander faced the dilemma of assisting them and allowing them onto their ship or not offering any aid. Vortena then started work on the Gravestone, and, after some hassle, got it operational.
Just as they were about to leave the planet, they came under attack from the Eternal Empire's Skytroopers and the Knights of Zakuul. Aided by Senya Tirall, who arrived by speeder bike, the group defeated the opposition. The Force-sensitives, Lana and Senya, used the Force to lift the Gravestone out of the muck, and they proceeded to leave the planet.
Concept art of Endless Swamp
The Endless Swamp appears in chapters four and five of Knights of the Fallen Empire, a 2015 digital expansion to the video game Star Wars: The Old Republic.