Agartha, also known as the Aether, Paradise, and the House is a dimension that exists outside of the known and unknown Universe. It is known as the first dimension and is the home of the Keepers, and the former home of the Apothicons.
In the beginning, the Keepers inhabited Agartha and created the Summoning Key to harness the power within the dimension. However, a group of Keepers discovered a dimension known as the "Dark Aether", opposite to Agartha. This dimension corrupted the Keepers, causing a schism in the group. Among these corrupted Keepers was the Shadowman. During the Shadowman's corruption, the being known as Monty became corrupted in an attempt to save his friend. The Keepers soon fell into civil war, marking the beginning of the War of Agartha. After the war, the corrupted Keepers were banished from Agartha to the dark Aether and eventually evolved into the malevolent race of the Apothicons.
Hellbent on returning to Agartha, the Apothicons planted Divinium meteors in different Dimensions in order to cause chaos amongst the humans that resided there. They also created a device known as the Aether Pyramid to harness the power beneath Agartha. Their plan worked, and a gateway opened in Earth, allowing the Apothicons to escape and attack mankind. However, 4 heroes known as Primis, alongside the Keepers, fought back the Apothicons in a terrible battle known as The Great War
Later, after touching the Aether Pyramid (now known as the MPD), Edward Richtofen became obsessed with finding Agartha and initiated a decade's long plan to take control of it. After being teleported by Richtofen, Samantha Maxis was teleported to the MPD where she entered it, and thanks to the request of her father, Doctor Ludvig Maxis, took control of the Aether.
At some point, Ludvig Maxis was transported somewhere in Agartha known as "The Crazy Place", where he learned to fuse his soul with electronic devices.
Richtofen later gathered "Tank" Dempsey, Takeo Masaki, and Nikolai Belinski to initiate his goals. Traveling through time, the group (known as Ultimis) gathered the Vril Device and Focusing Stone to eventually allow Richtofen to usurp control of the Aether from Samantha, switching bodies with the little girl. His contact with Agartha, however, was cut short when Maxis fired rockets towards the Earth, destroying it.
Richtofen and Maxis would later attempt to recruit a group known as "Victis", consisting of Samuel J. Stuhlinger, Abigail "Misty" Briarton, Marlton Johnson and "Russman" to activate three pylons to grant them power over the Aether's energy. The group followed the instructions of Maxis, who then used the Aether to destroy the Dimension in order to reach Samantha. However, Maxis was directly teleported to Agartha by Doctor Monty and promptly destroyed.
After the events of Buried, Samantha is sent through space and becomes imprisoned in Agartha in a realm known as "The Crazy Place", but is freed thanks to an alternate dimension version of Ultimis, that would later become Primis. This dimension's version of Maxis' brain is sent to Agartha in the form of a drone.
At the request of Doctor Maxis, Primis Richtofen then gathers a soul of his alternate self-using the Summoning Key, and the alternate selves of Dempsey, Takeo, and Nikolai. These souls are then brought to the house in Agartha.
Primis arrives in Agartha, but unfortunately brings the Shadowman (who was trapped in the key) with them, possessing Maxis' body and unleashing the Apothicons in Agartha. At the behest of Doctor Monty, Primis defeats and destroys the Shadowman, with Maxis and Sophia using the Summoning Key to destroy the Apothicons once and for all. Doctor Monty then sends Primis back to the Great War, completing the cycle of the universe.
After collecting Ultimis, Primis and their alternate counterparts travelled to the Broken Arrow facility Camp Edward in Nevada in an attempt to gain the Elemental Shard. There, they encountered and reactivated the A.I Rushmore. After completing various tasks, the crew eventually is led to the A.P.D, where they find Cornelius Pernell, the head of the Broken Arrow program, who has been turned into an electrical being after entering the A.P.D. Calling himself the Avogadro, the deranged Pernell threatened to kill Primis and Ultimis, and "ascend to Agartha". However, the crew gained the upper hand, defeating the Avogadro and sending him near the Hanford Site in Washington. In Agartha, Maxis brought Samantha and Eddie to the teleporter in the basement, and teleported them to Camp Edward to rendezvous with Primis. However, Doctor Monty entered the basement and revealed his true form to Maxis. Maxis proceeded to activate the teleporter, sending the children away from Doctor Monty's grasp. Enraged, Doctor Monty attacked and began to devour Maxis, killing him. When Samantha arrived at Camp Edward, she demonstrated her powers in a fit of anger over the loss of her father, destroying the camp. Samantha proceeded to give a speech to Eddie, promising that they would fight the Great War, return to the House, and kill Doctor Monty once and for all.
When Victis was teleported to Siberia by Ultimis Richtofen, he instructed the crew to build an "Agarthan Device". After the construction of the device, it is brought to Primis Nikolai, who uses it to destroy the Summoning Key, plunging the Multiverse into the Dark Aether. Along with every other dimension, Agartha, as well as Monty, was banished to the Dark Aether forever.