The first Legend of Heroes game was the sixth Dragon Slayer series game. Earlier Dragon Slayer games included Xanadu (which itself had three spinoffs that released in the US, Faxanadu on NES, Xanadu Next on mobiles and PC, and Tokyo Xanadu on Vita), Legacy of the Wizard (Dragon Slayer IV) on NES, and Sorcerian (Dragon Slayer V) on PC. Dragon Slayer: The Legend of Heroes was localized in English on the TG-16 CD but didn't really make a splash there.
Faxanadu and Legacy of the Wizard actually weren't bad games back in the day. For NES games they had a lot going for them. You can actually get Legacy of the Wizard as part of one of the recent Namco collections on Switch. Can't really say about Sorcerian since I never played that one.
I remember playing Legacy of the Wizard back in the day, and I didn't really care for it. Faxanadu...I remember playing, but I don't remember if I liked it or not. It probably wasn't either terribly good or bad, because I would have remembered it more otherwise. Neither of these games are anything special. That much I know.