Thunderbolt is a real-time RPG game available for MSX. I get the impression that it took a lot of their cues from Hydlide. The combat system seems basically identical. But graphically it sets itself apart to a great degree; your character and enemies are much larger, movement seems overall smoother, but it sacrifices a lot in the way of detail.

It has the concept of "underground", but the implementation seems a little strange. There are large pits throughout the land, and some of them you can enter. When you enter one, you seem to fall down as if you were taking damage, but it doesn't take away from your health. In all of the pits that I've entered, they are only single-room dungeons with a random number of enemies waiting for you.

One unique mechanic of gameplay is that a lightning bolt will suddenly crack on the screen, then what was once land becomes water. I am not sure if that was just to introduce variety or if there is some way that the normal world and water world interact.