My Harn players just finished with my own version of Harnic Ravenloft.
They are unusual for Harnic characters and campaigns though - I actually had to boost Strahd to provide a proper context for an "unkillable opponent" except via the path of the adventure. I turned Strahd into the god of his own sealed world keeping the populace terrified to fuel his powers by the time and depth of their fear.
The players though, are not D&D adventure players by background though (though their characters have been killing undead in Waterdeep of late and have gone through the "Throne of Bhaal" storyline and are effectively demigods themselves). They refused to follow any sort of intended adventure path and decided to walk in and bash the bad-guy head on.
The result was classic. Strahd said something like, "What you expected to march in here with a magic sword and kill ME
?" at which point he threw one up against the wall, teleported two into the dungeon (killing one and turning him into a vampire) and the last escaped.
Eventually all 4 escaped or were rescued and they came back the second time with a great deal more respect and preparation.
For all these adventures I have had to extensively modify the mechanics but they are a rich source of ideas and background. I found the 3rd edition D&D version of Ravenloft to be richer in detail but ultimately unuseable in terms of the creatures added to the module. The original Ravenloft version still needed modifications but was much easier to use.