Czech Republic

A small Eastern European country (but one leaning significantly into Central Europe) with a rich and chequered history, especially in the 20th century, which puts it on the dark tourism map too.
Czecheslovakia, as it was then called, was one of the first victims of Nazi
Germany's expansion in the late 1930s. Tolerated by
Britain and
France through the
Munich Agreement,
Hitler was able to swallow up the Sudetenland unchallenged, and when
WWII came, the whole Czech part of the state was declared a German "Protectorate" (i.e. was de facto annexed too) while the Slovak part became a Nazi-aligned dictatorship in its own right.
And with the Nazis came persecution and extermination – especially for Jews ...
the infamous Theresienstadt ghetto and concentration camp is the grimmest vestige of those days of the Holocaust. Another WWII atrocity site is that of Lidice, a village wiped out by the Nazis in revenge for the assassination of Reinhard Heydrich. Today, both sites are national memorials. And in Prague you can visit the site where the Heydrich assassins were finally tracked down and found their "martyrs' deaths".
During the
Cold War, the country – rejoined with Slovakia in the form of the the
CSSR – was part of the
Eastern Bloc. As such it had its fair share of communist repression, especially in 1968, when it was invaded by
Warsaw Pact troops in order to crush the
Prague Spring and restore compliance with the
USSR. However, the CSSR's transition to post-
communist democracy in 1990 was the swiftest and most peaceful in all of the former
Soviet satellite states (known, therefore, as the "
Velvet Revolution").
For the dark tourist, the grimmer aspect of repression under communism is represented by the former labour camp of
Vojna, now a memorial. The "
Communism Museum" in
Prague also touches on other aspects of the era, including some of the lighter sides. Prague's
nuclear bunker offers a bit of both
– the grim and the black-humoured.
A gem of an "artistically dark" (and totally non-political) older site is the magnificent
Sedlec ossuary, possibly the best example of its kind in the world.
The capital city,
Prague, steeped in history, is one of the world's premier tourist city destinations. For the dark tourist it is also a natural base, as the other dark sites in the country can easily be reached as day excursions from Prague … and the city itself has a few dark sites to offer as well. The second city of the country, Brno, is also well worth a visit, in particular for its underground sites.
UPDATE: officially, as part of a PR move, as I've understood it, the county has renamed itself simply "Czechia". However, I won't go through this website and change every mention of the country to this simpler, shorter form, but for now I will just stick with "Czech Republic".