Here's how it works. You travel to Riverwood's Sleeping Giant Inn, preferably with a brand new character who has skipped past the tutorial thanks to Alternate Start—Live Another Life (opens in new tab) or your preferred alternative. You track down the ghost lady in the corner of the bar and talk to her, and then you get whisked away to Transylvania, as interpreted by Castlevania 2: Simon's Quest and now recreated as a Skyrim mod.
Yes, you get a chain whip. The vampire killer weapon will be your best friend as you whip through the various skeletons, werewolves, and mermen that you meet on this questline (and who all drop hearts when they die, which you collect with a glorious beeping noise).
It's pretty straightforward. In towns you talk to people and visit the church, hoping to find someone who will let you trade hearts for an upgrade. Then you march out into the spooky wilderness again, preferably in daylight, to whip your way through monsters on your way to the next town and the next objective in your quest to return Dracula to the grave.
Though of course it's not a 2D platformer, the CV2 mod does remain faithful to its inspiration in a lot of ways. Skyrim's regular levelling system is gone, and instead you trade hearts to priests for level-ups. They'll also heal you as well, which is nice because the regeneration over time that's normally part of Skyrim is also gone here.
Another part of the original game preserved in this mod is your inability to swim. Touch the water in Transylvania and you'll die instantly. That makes the section where you have to cross a broken bridge while being shot at by mermen particularly annoying. First-person platforming is the worst, and in Skyrim's third-person mode it's only slightly better. I've drowned a lot more than I've been killed by werewolves.
The CV2 mod's an impressive thing overall, featuring a whole bunch of new music too including remixes of Castlevania tunes. You can download it for Skyrim Special Edition here (opens in new tab).