Here's another view of it, and in the comments is a further link to that photographer's gallery of it. It's called the Buzludzha building and monument, built in 1981, and served as the symbolic headquarters of the Bulgarian communist party. Despite not being terribly old, it seems to be in pretty poor shape, and heavily graffitied. The roof was one of the first things to go - apparently it used to be entirely copper but was looted within the first night that there weren't security guards watching the place (which was already a few years after the building was abandoned, if I understand correctly).
The building also features in the photography book Soviet Ghosts by Rebecca Litchfield (who now goes by Rebecca Bathory, and has an excellent history with even more abandoned places) -
Soviet Ghosts: The Soviet Union Abandoned: A Communist Empire in Decay https://www.amazon.com/dp/1908211164/ref=cm_sw_r_cp_tai_movSyb22ZGXJW