Castlevania II: Simon's Quest
A funny thing happened between 1987 and 1988: a bit of Metroid got mixed up in the Castlevania series. Now, we're not talking about Simon Belmont going off the fight Mother Brain (although we at the Inverted Dungeon think that would be an epic adventure worthy of a proper release). No, what happened was that Simon Belmont was freed of the confines of Dracula's Castle and given the countryside of Romania to explore. This all came together in Castlevania II: Simon's Quest (Dracula II: Noroi no Fuin in Japan, translated as "Dracula II: Seal of the Curse") for the NES and Famicom Disc System.
As it goes, Dracula was dead, defeated by Simon Belmont, but Dracula had his bits (nail, eye, ring, heart, and tooth) scattered across Romania. A curse was placed on Simon that would kill the vampire hunter if Dracula's body parts weren't put back together (ensuring Dracula's resurrection). To find Dracula's body parts, Simon had to adventure through a variety of locales -- forests, caverns, graveyards, and mansions. Along the way he could upgrade his equipment (multiple whips and daggers, as well as a variety of other equipment actively-used weapons and latent bonus equipment), find important items, and even level up.
Of course, saying that "Konami mixed Metroid in" really doesn't explain the whole story. While Nintendo was working on the first adventures starring Samus Aran, Konami was experimenting with their own exploration-style adventures. The company created titles like Knightmare II and The Goonies that were exploring the nascent genre in its own way. The Metroidvania genre hadn't yet gotten that name (and wouldn't for another decade), but the concepts of those games, and what would become a defining play style for the Castlevania series, were already getting set in place.
While it's easy to see the bones of Simon's second adventure in these other titles (the development team on Castlevania II stated they drew direct inspiration from The Caves of Galious: Knightmare II), it still does seem like an odd game if all you have are the other Castlevania titles to compare it to. The game is very different in concept from the original Castlevania, with a more diverse, more fleshed-out experience. Gone was the linear path through a castle, replaced with wide open world filled with mysteries to solve, ruins to explore, townspeople to talk to, and quest items to find. This was a different kind of game from the originating title, making it something of a black sheep in the series, especially in the West where we didn't see many of Konami's exploration-based adventures.
Likely because of this, as well as decreased sales for the game, the series went back its more traditional roots after Castlevania II. It focused much more on the heroes in leather kilts going on a more-or-less linear quests through Dracula's castle, settling into that much more familiar groove from the first game for many of the future titles during the NES and SNES era. Konami did continue to explore this avenue of play, just in other games (like Getsu Fuma Den and Konami Wai Wai World), but the Castlevania series remained much more linear for some time.
At least until Castlevania: Symphony of the Night came along and really helped cement the rise of the Metroidvania genre. Now Castlevania II seems more like an early precursor to where the series would eventually end up, and not just this strange one-off (or two-off, counting Vampire Killer) in the early era of the series.