The subject of this article or section originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
Seregost is non-canonical, as it is not from the works of J.R.R. Tolkien. It was created by the now-defunct Land of Shadow website, and has since been adapted by licensed games such as Middle-earth: Shadow of War and The Lord of the Rings Online.
Seregost was a dark fortress in the Mithram Gap, with the Plain of the Black Steeds to its north. The Seregost Road led to it from Barad-dûr, on its way to the Great Eastern Road. It was a much traveled road in Mordor, that lead to the desolate East. Along the road came the armies of the Easterlings. Seregost Road ran over the gap in the Mithram Spur, and then branched out to the East Road, and the Caran Road to the south. It was a vital route in Mordor.
Seregost was a place of Dark sorcery and upon its molten foundations a great fortress stood that guarded the Eastern Road.
Shadow of War
Seregost is a region within Mordor. Despite its proximity to Mount Doom, it was bitterly cold and snowbound no matter the season.
It is the location of the fortress of Khargukôr where many Black Uruks dwelt.
The region is comprised of steep mountain passes, often with sheer drops or curving roads, and includes several icy passages and a large frozen lake, nearby where many Frost Graugs can be found along with Caragors.
After the first downfall of Sauron at the end of the Second Age, the Uruks took this Fortress and inhabited it for many centuries until their master’s return. It was a place of Dark sorcery, with molten lava covering the landscape.
The Lord of the Rings Online: Mordor
It is a tower within the corrupted swamp of Agarnaith, which lay in a valley directly east of Mount Doom, and southeast of Barad-dûr. The fortress of Seregost is found in the north-east of Agarnaith, straddling the river Chaydash. It divides Agarnaith proper from the Mokál Rukh, and is said to be accessible only by flight. The Conquest of Gorgoroth will have to find another way of bypassing its sheer western walls. The mighty fortress of Seregost, citadel and headquarters of Lhaereth the Stained and her Stained Merrevail, dominates the land and blocks access to the even more deadly Mokál Rukh beyond. Guarding Seregost is the lesser stronghold of Kala-gijak, held by Uruks.
Seregost has weaknesses to exploit, however. Anyone clever and nimble enough might find a way to climb up, though it may seem impossible to those lacking in resourcefulness. The pair of Watching-stones on either side of the fortress share a mysterious bond and are no friends of Lhaereth. And anyone approaching Seregost from the east will find two entrances at ground level, each guarded only by a pair of Uruks.
The halls leading from these entrances have openings on either side, about halfway through, providing access to the Western and Eastern courtyards and to the centre of the fortress. In the centre is a door to the Pits of Seregost, where Lhaereth's prisoners are kept. However, this is only accessible during The Secrets of Seregost. At the other end of these halls, opposite the entrances, are the Western and Eastern Terraces. Stairs in each courtyard go to the upper level, where the Mokál Rukh Terrace looks out to the north-east and the Overlook Terrace looks over Agarnaith to the south-west. On the Overlook Terrace is the entry to the Court of Seregost, the chambers of Seregost's guests and leadership.
The swamp is full of poisoned and corrupted orcs, trolls, bats, beasts, huorns, ill looking plants, and fungus corrupted from the foul run-off and pollution from the tower. The tower of Seregost itself is ruled over by Morival Queen, who used to make poisons, plagues and other concoctions used by Sauron's forces. A smaller fortress containing Uruks and trolls can be found on its southern edge. The corruption to the swamp by the pollution from Seregost citadel, has caused the land to turn blood red (hence how the fortress received its name).
Seregost is Sindarin for 'Blood-fortress'.