Gargoyle's Quest by Capcom for the Game Boy, released in 1990.
Well, then! Culled from the pages of Ghosts 'n Goblins, one of the later and greater Capcom series from back in the day, we have a new adventure starring.. a bad guy? That's right - rather than playing as noble Sir Arthur in search of his maiden (and a change of briefs), in this game you command an evil demon from the ghoul realm.
It would seem the tables have been turned, however - as you and your cohorts were previously occupied invading the normal world from your own demon-infested hell dimension, an even worse plane of existence is infesting your own already-evil place. Oh, how many levels deep can it go! So, you are tasked with destroying all that is gnarly and unholy in your land of.. horrible, unspeakable evils.
The game was quite a departure from it's predecessors' platforming roots. You travel around in an overhead mode, visiting towns and such, and doing the usual RPG-lite tasks. When it's time to battle, the game becomes a sidescroller and your puny, weak demon must take on all manner of irritating characters. Fortunately, as the game progresses your powers do the same, and after a good few levels in you have a pretty formidable arsenal.
Gargoyle's Quest was really an interesting "side-story" take on the whole Ghosts 'N Goblins mythos, and a couple of other decent games followed on the big brother consoles. This is where it all started however, and all these years later it's still a worthwhile game to pick up and play.