There probably isn’t a direct relationship between them. The original I6 Ravenloft module came out in 1983. The original Castevania game for the NES came out in 1986. On the one hand this was the 8-bit era, so it’s likely Castlevania began its development after Ravenloft had come out. On the other hand pen-and-paper RPGs were a small-scale hobby and no one had casual access to the Internet back then, so it’s unlikely that Castevania’s Japanese developers would have heard of the module.
Still, both Ravenloft and Castlevania very clearly get inspiration from the same sources, and tell remarkably similar stories: one or more intrepid adventurers enter a gloomy kindgom ruled by an ancient and cruel vampire lord, break into said lord’s castle, and kill him with a weapon destined for the purpose1.
The environments in both are heavily inspired by gothic literature and monster movies. As games, both have a reputation for being quite hard. Strahd from Ravenloft is clearly inspired by the literary Dracula, with a different name to make him more copyrightable. Konami went the other way and named their vampire Dracula while making him completely different in most other aspects.
I mention the original I6 adventure specifically because I think Ravenloft loses quite a bit of its Castlevania-ness when it turns into a campaign setting with a metaplot and that business with the Mists. The original adventures set up dynamic scenarios: things are bad, and have been for a while, but the PCs are fully expected to upset the apple cart as soon as they get into the picture. The Ravenloft campaign setting, on the other hand, is just another static D&D setting where the only allowed changes are handed down from on high in the form of metaplot.
Í haven’t really read the new-ish Curse of Strahd sourcebook2, but I gather it’s tries to turn I6 into a full campaign starting the characters at level 1 and lavishing praise on Strahd himself at every opportunity. Since I didn’t really intend to use the rules as printed on either version I went with the cheaper one.
There’s no deeper insight here. Just something I’ve been thinking off and on for a while. I’m obviously not the first person to think of this: someone home-brewed a campaign setting called Barovania back on 2012 or so which mixed a lot of old-school Nintendo lore with the usual old-school D&D soup. Finding a link to that now is a challenge, but I remember reading it.
In any case, if you want a Castlevania experience in your tabletop, you can pretty much run I6 Ravenloft as written and just convert the monster and item stats to your system of choice. Maybe use the Castlevania soundtrack as mood music. There are quite a few organ arrangements for it too. You could also forgo the random placement of the special treasures and make it so one of them is in a place that’s easy to see but impossible to reach without first finding the other.
For extra franchise power, after your group beats Strahd, run Ravenloft 2: The House on Gryphon Hill as a sequel by having it take place 100 years later and feature the descendants of the PCs in the first adventure. All the thematic stuff you need is already in there and pretty system-neutral.
You can technically kill Strahd without finding the Sun Blade first if you’re badass enough, but doing things in the “proper” order is both easier and more dramatic. ↩
Curse of Strahd is only available in print, which makes it hella expensive. I6 and Gryphon Hill were available as PDFs for much, much less. ↩