Xevious: Fardraut Saga is a shooter for the MSX2 and Xevious: Fardraut Densetsu (ゼビウス ファードラウト伝説?) for the PC Engine are two related games developed by Compile and published by Namco, which are part of the Xevious series. It is likely that the later Fardraut Densetsu, being fairly similar and made for more powerful hardware, is an "upgraded version".
Fardraut Saga
Players can select between two modes at the title screen: "Recon", a port of the original Xevious; and "Scramble", which is a new 16-area game with new enemies, new boss battles as well as the Andor Genesis, and four ships (Solvalou, Solgrado, Zeodalley, and Gampmission).
Fardraut Densetsu
The two modes have been renamed to "Arcade" and "Fardraut". However, Fardraut is a bit different from the original Scramble: it is a 4 stage game with new enemies and a different ship on each level. At the end of each level, a boss battle which is similar to the Andor Genesis will be at the end of each level. Before each level players will be given a dialogue of the story that is given in the game.
Translation note
While the two games are advertised with the phrase "ファードラウト伝説", the games themselves bear two different names, although "saga" can be taken as a translation for "伝説". If so, both games would be "Fardraut Saga", although the PC Engine version never makes use of such a translation anywhere.