It commemorated the events of 1891, when a group of socialists led by Dimitar Blagoev assembled secretly in the area to form an organized socialist movement that led to the founding of the Bulgarian Social Democratic Party, a forerunner of the Bulgarian Communist Party (itself a forerunner of the current Bulgarian Socialist Party).

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The monument exemplifies the futurist architecture common to many state-constructed communist buildings. All maintenance ended with the fall of communism in 1989, and the building remains closed to the public due to the hazards of the weakened structure.[1] The Buzludzha Project has helped to begin work on the monument's preservation, with the eventual aim of creating an interpretation center for Bulgarian history.[2]

https://en.wikipedia.org/wiki/Buzludzha_monument

More pictures on that wikipedia page, very interesting. The site of the Battle of Shikpa Pass (from the Russo-Turkish War) is just a couple miles away too.