Castlevania: Chorus of Mysteries is a Castlevania 1 hack. Here is a little info from the Official Site:

Chorus of Mysteries is an extremely indepth rom hack of the original Castlevania for the NES. This hack edits the graphics, and level layouts. It also includes a lot of things never before seen in the original. The new things include new enemies, new bosses, a completely new soundtrack, and other new added gameplay mechanics.