The Famicom Disk System version of Castlevania was the very first Castlevania game ever released, it being published by Konami in 1986.
It began a long-running series of platform/horror-themed video games and set the template for the Castlevania series as a whole.
You play the infamous Simon Belmont, a whip-wielding monster-killer on a mission to defeat Count Dracula in his castle. Simon must avoid the many monsters that attack him by either hitting them with his increasingly powerful whip, or by using special, limited weapons that can be picked up as he progresses.
Castlevania was such a hit on the Famicom Disk System that it was re-released on cartridge for the NES in 1987. It has also been re-released and re-made numerous times for a variety of different platforms.
More: Castlevania on Wikipedia