Ravenloft

Those who venture into Ravenloft enter a setting filled with dark terror and a mysterious demiplane of horror. This is not a specific planet or continent, but rather a network of countries interconnected by a mysterious, impenetrable fog – the so-called domains.
Each domain is the personal prison of an evil being, the so-called “Dark Lords” – particularly malicious and corrupt creatures and humanoids from across the multiverse, imprisoned there by the mysterious “Dark Powers”.
The most well-known among them is probably the vampire lord Count Strahd von Zarovich, who even has his own campaign book with “The Curse of Strahd”. He rules over the domain of Barovia, and his story is strikingly reminiscent of a famous vampire’s.
The Dark Lords cannot leave their own domains and entering them is also difficult, which is why the web around Ravenloft is especially suitable for shorter standalone sessions. The reason for this is the dense fog that forms the boundaries of the individual domains. Once entered, they spit you out at a completely different location.
They cannot be controlled, nor can they let you escape unless the “Dark Powers” allow it. So, if you are looking for something dark with heavy moral decisions, you may find yourself happy in Ravenloft.