The Dark Aether is a dimension that houses the ancient, evil race of the Apothicons. It has the ability to corrupt anything that enters the dimension.
As of Tag der Toten, the entire multiverse has been plunged into the Dark Aether.
At the beginning of time, the Keepers created the Summoning Key in Agartha to create the universe. However, a group of Keepers, which included the Keeper who would be known as the Shadowman, discovered the Dark Aether, who wished to use it for their own benefit. As the Shadowman entered the Dark Aether, the being who would be known as Doctor Monty attempted to stop him, but was corrupted himself in the process. Sometime after this, the corrupted Keepers created a device known as the Aether Pyramid, that used the Dark Aether to give its user the power to control the Aether itself. When the rogue Keepers became corrupted, a war in Agartha broke out between the corrupted and non-corrupted Keepers. The non-corrupted Keepers won the war, banishing the corrupted Keepers to the Dark Aether. During their millenia-long banishment, the corrupted Keepers would morph into beings known as the Apothicons.
Noting the existence of humanity, the Apothicons sent Divinium meteors to Earth, hoping that they would use it to fight each other. Their plans came to fruition in 1292, where humanity's use of Divinium opened a portal from the Dark Aether, allowing the Apothicons to invade. This would eventually spur into The Great War, where humanity assisted by a group of four mysterious robed warriors (known collectively as Primis) wielding Elemental Staffs. The war would end in 1299, with the Apothicons being once again banished to the Dark Aether.
Sometime later, a German scientist known as Edward Richtofen made contact with the Aether Pyramid, and began to hear the voices of the Apothicons (who were still within the Dark Aether) in his head, who encouraged him to enter the pyramid.
In another dimension known as Dimension 63, the Shadowman manipulated a reporter into retrieving the Summoning Key before the reporter was killed by an alternate version of Richtofen. In the same dimension, Apothicons manipulated 4 individuals within Morg City, Nero Blackstone, Jack Vincent, Jessica Rose, and Floyd Campbell into using the Summoning Key to open a portal from the Dark Aether into Dimension 63. The scheme succeeds, and the Apothicons invade the dimension, however, the Shadowman is trapped within the Summoning Key by the 4 individuals after the initial invasion. The Apothicons eventually consume and destroy the Dimension entirely.
When Primis arrived in Agartha, the Shadowman tricked Ludvig Maxis, who inhabited Agartha at the time into granting him a vessel. After being freed from the Key, the Shadowman opened a portal from the Dark Aether into Agartha. Unfortunately for the Apothicons, Monty uses Primis to defeat the Shadowman, who subsequently use the Summoning Key to plunge the Apothicons back into the Dark Aether and to close Agartha off from the rest of the multiverse. Monty then sends Primis to the Great War, ending the loop of the cycle.
In another version of the cycle, the Warden of Alcatraz is manipulated by the Shadowman from the Dark Aether into using Alcatraz as a trap to use Primis Richtofen's blood to open a portal to Agartha. The scheme fails, and the Warden is killed, and the cycle is broken.
After being given leadership of the crew by Richtofen, Nikolai Belinski, aided by Ultimis, Primis and Samantha Maxis gather the Elemental Shard, and use a group of civilians known as Victis to gather the Agarthan Device from a base in Siberia. They then use both the Device and the Shard to destroy the Summoning Key, plunging the multiverse (including Monty, Agartha, and Victis) into the Dark Aether, solving the Paradox of the multiverse.
Almost immediately after, the universe Primis and Ultimis were in is plunged in to the Dark Aether. Still alive, Samantha and a child version of Richtofen known as Eddie hold hands and walk out of the Dark Aether into a new, uncorrupted universe.