It was included in compilations for the MSX, PlayStation and Sega Saturn, followed by a port for mobile phones, and digital re-releases for the Virtual Console and Microsoft Windows.

Knightmare is a vertical-scrolling shoot 'em up game starring Popolon, a warrior who embarks on a quest to rescue the princess Aphrodite from the evil priest Hudnos.

The player controls Popolon through eight increasingly difficult stages across a Greek-esque fantasy setting, populated with an assortment of enemies and obstacles, over a constantly scrolling background that never stops moving until a boss is reached, which must be fought in order to progress further.

[1][2][3][6] These can be cycled through other weapons and power-ups by firing at them, ranging from boomerangs, three-way shots, swords, speed increasers, temporary invincibility and a shield to withstand enemy projectiles.

[4][7] Getting hit by an enemy or blocked by an obstacle and scrolled offscreen will result in losing a live stock, as well as a penalty of decreasing Popolon's overall firepower to his original state.

Knightmare, known as Majou Densetsu in Japan, was developed by the MSX division at Konami under management of Shigeru Fukutake, who revealed about its creation process in a 1988 interview with Japanese publication Micom BASIC Magazine.

[12] Masaaki Kukino (of Haunted Castle, Asterix, and Silent Scope) revealed that the game's title is a portmanteau of knight and nightmare, giving it a double meaning.

[16] Konami also released a slightly altered version that could be downloaded from the "LINKS" network (a Japanese equivalent for internet in the 1980s), where the player could submit high scores to an online server.

[17] Unlicensed versions for the SG-1000 and arcades were also distributed under the names Mó Yù Chuánqí (魔域伝奇) and Pesadelo ("Nightmare" in Portuguese) by the Taiwanese company Jumbo and Brazilian manufacturer Fort II respectively.

[18] Its main theme was featured alongside music tracks from Penguin Adventure and King Kong 2: Yomigaeru Densetsu in a compilation album titled The Konamic Game Freaks, distributed in Japan by Alfa Records in 1987.

[29] Aktueller Software Markt's Wolfgang Rui noted the game's challenging difficulty but expressed admiration towards the music, stating that it was "the best thing i've heard so far on MSX".

[18] Retro Gamer's Stuart Hunt highlighted Miki Higashino's music for enhancing the gameplay, but found the game difficult due to the lack of save options.

[38] MeriStation's David Pérez García and Francisco Alberto Serrano praised its graphical department, lack of slowdown, soundtrack and controls, deeming it as a "must-play" for shoot 'em up fans.

[43] IGN Spain's Jaume Esteve described it as an unusual vertical-scrolling shooter due to the absence of ships and compared its power-up system with that of TwinBee, writing that "despite the technical limitations of the computer that were mainly reflected in that limping scroll, Knightmare deserved much more impact than it finally had, although it is certainly one of the games that are essential when it comes to touching the catalog of the MSX.

[1][18][39][44] In addition to the main games, a stand-alone sequel titled Majou Densetsu II: Poporon Gekitou Hen[b] was scheduled to be published in 1987 by Konami for the PC-8801 and X1 home computers.

[1][18] In 2005, a mod dubbed Knightmare Gold was released by Portuguese fan group Amusement Factory, which allowed support with the MSX2 hardware for smooth scrolling, an upgraded color palette, among other changes.