The subject of this article originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
Seregost was a fortress in central-northern Mordor just south of the Ered Lithui, on the Mithram Spur. It is said that the Black Uruks were bred here, back when Sauron used them to assault Ithilien before the Great Years.
Seregost is Sindarin for "Blood-fortress"