The Castlevania Adventure
100 years after Dracula's defeat at the hands of Trevor Belmont, the Dark Lord came back again (well-rested and a touch peckish). Once again, Dracula had designs to take over Romania (and eventually the world), and refused to let anything get in his way.
Except, of course, there was a Belmont ready to kill the evil vampire and send him back to the grave. Christopher Belmont, heir to the clan, readied his trusted whip, the Vampire Killer, and set off to Dracula's castle. The battles were fierce, the dangers numerous. The hunter reached the top of the castle where he faced off with the Dark Lord, and in the end, the Belmont walked away the victor.
For a time...
This game is a solid illustration in why old NES manuals could never be trusted. The manual for this game in America was vague about who was fighting Dracula, but the implication was that this game took place after Simon's games, actually making this allegedly a sequel.
In Japan, though, this was always intended as a prequel. It's actually the first to establish (via the Japanese instruction manual) the fact that Dracula comes back every 100 years, thereby setting the game over 100 years before Simon's era.
This was only exacerbated when Castlevania III came out, as the manual there stated that Trevor's game was 100 years before Simon's (it actually takes place 200 years before, making it 100 years before Christopher's adventure). Eventually fans just decided Christopher's games happened after Simon's, removed by another 100 years.
Really, the misconception wasn't cleared up definitively in the US until the internet made it much easier to share information with the greater fandom.