A hybridized remix of Castlevania III: Dracula’s Curse and Akumajō Densetsu, the original domestic release.
For those not in the know (such as me a few hours ago until I saw Frank Frank tweeting about it and saw someone link this in the comments), the Famicom Disk System was an add-on for the Famicom which allowed the Famicom to use floppy discs. This was eventually made obsolete by improvements in semiconductor technology in cartridges, but it also meant that some games that the development of was enabled by the add-on had compromised releases in North America, which did not get a Disk System of its own. This included a whole extra audio channel, for one, so for instance like in Frank’s tweet, some soundtracks suffered in particular without that extra audio channel.
Some soundtracks, though, were made with greater care than others, which some people contend for Castlevania III: Dracula’s Curse, which some say offers some advantages over the original with its one more audio channel and high fidelity audio. EDIT: correction, it has 3 extra audio channels!
So, because people who really like videogames are unstoppable, someone made a hybrid mix, and attempted to create a Akumajō Densetsu/Castlevania III: Dracula’s Curse hybrid mix, highlighting the strengths of both.
It’s not a real game soundtrack of course, but man, there are some wonderful sounds in here.