We did a lot of research for our Rastan article, surely this isn't something we missed? So I tried to dig up any info I could glean from this tiny preview - that it's a Gameboy RPG made by Taito, and stars a character named Will (as evidenced in the screenshots, though they're in Japanese.) This turned up a game called Knight Quest, which was released in both North America and Japan under the same title.
While published by Taito, it was developed by Lenar, who also did *shudder* Deadly Towers. It's a pretty bog standard RPG similar to Dragon Warrior, in that you're a single character that roams around the landscape, fighting turn-based battles and grinding for EXP/cash. The only major differences are that enemies are visible on the field, and the side-view battles that make everything look a little more action-ish. It certainly doesn't seem to have very much in common with Rastan - the world and enemies are more like generic-RPG than anything similar to Conan or any other source that Rastan ripped off. So what's the deal?
(A) This was originally attached to the Rastan IP at some point in development, as sort of a Rastan Jr. meant for kids and positioned as an RPG. Not unfeasible, considering Sega did something similar with Golden Axe and its Game Gear spinoff Ax Battler. Plus, they shoehorned the Rastan character into the Western-developed arena fighter Barbarian, although that happened well over a decade later. Or...
(B) Electronic Gaming Monthly was mistaken and was talking out of their asses. One of the most fun parts of reading old EGMs is reading previews of Japanese games and trying to pinpoint where the writer had no idea what they were talking about and was just rambling off some text based on a few screenshots. Based on the vague preview - "Rastan plays unbelievably well considering the small screen and lack of color. The quest is long and the game play is challenging" - this may very well be the case.
There's no title screenshot in the EGM, alas. And so there's no real definitive answer we can dig up, but it's interesting to ponder. Anyway, you can read more about Knight Quest at the excellent Gaming Sanctuary.