Akumajou (Akumajo) Dracula X: Gekka no Yasoukyoku is the Japanese version (only sold in Japan) of the Castlevania: Symphony of the Night (悪魔城ドラキュラX 月下の夜想曲) originally released on PlayStation (PSone, PSX) console. It was published as the direct sequel of Akumajou Dracula X: Rondo of Blood, for Sega Saturn platform.
Vampire Killer (full name: Akumajou Dracula - Vampire Killer) is the second game in the whole Castlevania series and its second work released in the same year 1986.
Castlevania 1986 (also known as CastleVania 1, Akumajou Dracula, 悪魔城ドラキュラ, Akumajō Dorakyura, or Devil's Castle Dracula) is the opening work of KONAMI's whole Castlevania series video game. First released on FDS (Famicom Disk System) platform in Japan in 1986, and was transplanted to NES (Nintendo Entertainment System) platform in EU/US later in the 1987 and 1988. So it also can be called Castlevania FDS/NES.