The short answer is that we don’t know what games Castlevania: Nocturne will take place around. Thanks for reading. BUT! We have some speculations based on the trailers.
Since Richter Belmont is center-stage in Castlevania Nocturne, that narrows it down to two titles: 1993’s Castlevania: Rondo of Blood (Akumajou Dracula X: Chi no Rondo) and 1997’s Castlevania: Symphony of the Night. Those are the only two games that Richter had a prominent role in, and has only made cameos ever since.
However, it seems like the first season is just going to cover Castlevania: Rondo of Blood. The evidence? Not a dhampir in sight. Castlevania: Symphony of the Night mostly starred Dracula’s half-blooded son, Alucard. He isn’t even hinted at in Netflix’s longest trailer.
Moreover, Richter’s companion from Castlevania: Rondo of Blood, Maria Renard, is shown in an outfit that is much closer to her original appearance rather than her more adventuresome look in Symphony of the Night.
Our conclusion is that Castlevania: Nocturne is going to be set around Castlevania: Rondo of Blood. That is, at least for the first season. It’s not unfeasible that Netflix will choose to continue on to the plot of Symphony of the Night as the show progresses.
