Tristan da Cunha
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The main island of a group of islands in the South Atlantic Ocean and part of a spread-out British Overseas Territory cluster which is administered from St Helena as the "capital".
The only settlement on Tristan da Cunha is officially called Edinburgh of the Seven Seas, but it is known locally simply as "The Settlement" – no need for complex place names when there's no other about! It is famously known as the most isolated community on Earth (at least as far as permanent settlements go). Less than 300 inhabitants live here in almost complete isolation from the far-away rest of the world.
St Helena is the "nearest" other inhabited island – a full 1500 miles (ca. 2500 km) away. The "closest" mainland city is Cape Town in South Africa, and it is from here that the island receives its supplies by cargo ships. The "homeland" and the Queen in London are 10,000 miles away.
In other words: it really doesn't get much more remote than this.(On account of sheer distance to other inhabited places, however, one contender is Easter Island in the Pacific, which is quite comfortably reached by plane from Chile, though.)
If remoteness isn't quite enough to qualify as a dark tourism destination, the fact that the island is also volcanic is. During the last major eruption in 1961 the entire island's community was evacuated to Britain, where they soon got unbearably homesick and campaigned to be allowed to return home, which they did from 1963 onwards.
Today the community is back in the old trot of fishing and growing potatoes (both allegedly yielding excellent produce).
The island's "sights" are naturally limited, it's mainly about just being in such an extreme location. Other than that there's rare bird watching (albatross!), hiking and climbing the island's volcano summit, St Mary's Peak, to see its unique heart-shaped crater lake. You can also explore the lava flow just to the north-east of The Settlement, which was the threat that caused the evacuation of 1961.
But is it at all possible to visit this ultimately remote island as a regular tourist (as opposed to a professional explorer)? Yes, but it's predictably anything but easy to organize. That is unless you are on one of those few cruise ships that occasionally drop by, mostly en route to South Georgia and/or the Falklands and Antarctica. But not all of these actually allow time for landing on the island – which may also not be possible for adverse weather conditions (which down there are as common as traffic jams in London or Beijing).
Getting to Tristan da Cunha independently and staying on the island for a (pre-determined long) while, though not totally impossible, is still almost an expedition, and requires a lot of preparation well in advance (at least a year) – as well as a special permit from the local authorities too. For more info see the "visits" section on the island's website (yes, it has one, and it is really quite informative!) at: tristandc.com.
There are only 10 scheduled supply ship journeys a year, with as few as 12 berth spaces for passengers – and demand is (perhaps surprisingly) high, so early planning and application is essential. Since there's no other transport (no airfield or anything) this is the only regular connection to the rest of the world.
The financial means required are actually not as excessive as one might expect, with return fares of 800USD, and accommodation costs between 20 and 40 GBP per person/night, self-catering or as homestays (with full board). It's rather the restricted access and the time factor that make the place a "luxury" destination, not what you'll have to pay locally or what you get for it.
Still, it's high on the list of many extreme world travellers, simply because it is so extremely extreme a destination.
Location: in the middle of the South Atlantic Ocean, almost 1800 miles (3000 km) west of Cape Town, South Africa, and 2600 miles (4100 km) east of Buenos Aires, Argentina.
Google maps locator: [-37.11,-12.29]
(The photo above showing Trustan da Cunha Newsletter editions was taken in the Museum of St Helena where several copies of these were on display when I was there in 2018).