What a horrible night to have a curse...
Castlevania II: Simon's Quest was developed and published by Konami for the NES in 1988. The game (as of this noding) is long out of print, but easily acquired on eBay or in any halfway-decent used game store. The ROM image is complete and emulates well. The packaging is simple, with the typical silver Konami box/label of the time, with Simon (in his red and black Vampire Killer outfit) facing the viewer, his whip mid-crack. (Don't confuse this game with Castlevania II: Belmont's Revenge, the sequel to The Castlevania Adventure.)
The Japanese version of this game, Akumajou Dracula 2: The Accursed Seal, was also available for the Famicom Disk System, which allowed for name entry and save files, but had some load time and inferior music.
The direct sequel to no less than four games, at least two of which would be released years later, Castlevania II is set seven years after the events of the original Castlevania. Dracula has been slain, but the effects of his presence are still felt, as a darkness is creeping over the land of Romania, and a curse claws at Simon Belmont's heart, slowly consuming him. Now, Simon must collect the five remaining parts of the dark Count's body (rib, heart, nail, eye, and ring) and revive his immortal foe in order to seal the vampire away for another century. (Of course, while the premise is well-articulated, the story consists of "Simon goes out and does this, goes home.")
That said, the only thing this game has in common with the original Castlevania is a whip-armed vampire slayer and Dracula. Even superficial details are changed, as Simon now wears his red and black outfit from Vampire Killer instead of his yellow and brown outfit. Rather than Castlevania's strict left-to-right platforming gameplay, this game is completely nonlinear, as Simon can wander as far as his skills at the moment can take him. Almost completely missing are the subweapons (relegated to being pretty useless items; see below), and whip powerups are instead permanent and purchased from shopkeepers.
As the premise would imply, the game is focused on collecting items, most of which are used to overcome the game's roadblocks. However, rather than a Metroid-style arrangement (as would be seen in Castlevania: Symphony of the Night), the world is arranged left-to-right, with towns full of people walking back and forth (and women who curiously all seem to hit on Simon), stretches of wilderness full of skeletons and werewolves and such, and mansions full of sundry monsters all guarding a piece of Dracula. Should you need to stop, the game offers a decent (if slightly annoying) password system.
Of special note is the progression of time in the game, something rare in games of the period. (The best-known other example would probably be Ultima: Exodus.) As Simon travels in the wilderness, time passes, going from day to night and back again. (The immortal quote at the top of this w/u is the player sees as the sun sets.)
The game has three endings. The easiest to get is, curiously, not the worst ending; it's a shot of Dracula's grave at nightfall, with a short scroll about how Dracula's curse is overcome and Simon is a hero. The worst ending, which has to be earned by finishing the game with few continues, but in more than eight hours, has Simon kneeling at Dracula's grave on a sunny day, with a scroll about how Simon defeated Dracula, but later died of his fatal wounds. The best ending, earned by finishing the game with few continues and under eight hours, is Simon kneeling at Dracula's grave at sunset, with a scroll about Dracula's curse receding from the land.
Why bother going back to Castlevania II? For one, the music. This game's soundtrack, composed by James Banana, stands up against any other NES title, and really wasn't surpassed until the musical masterpieces of the SNES, like Actraiser or Final Fantasy VI. There's also the Engrish, if you're into that. Every so often you'll hear things like "YOU NOW PROSESS DRACULA'S RIB" or other such nonsense.
As for how Castlevania II fits into the rest of the Castlevania series, it's the last appearance storyline-wise of Simon Belmont, the best known of Dracula's many rivals, as well as one of only five vampire hunters to make multiple appearances in the Castlevania series (and the only one to make three appearances, as well as the only one to appear twice as the main character). Simon's legacy would inspire Maxim Kichine to attempt to undertake his quest in Castlevania: Harmony of Dissonance (and Juste Belmont would end up destroying Dracula just as Simon did). He was also the first, although not the last, vampire hunter to be corrupted by Dracula's influence, although he was the only one to throw off the curse on his own.
Castlevania II drew heavily on Vampire Killer in its nonlinear style, although it turned out to be a dead end in the evolution of the series, as, while it sold decently, it was not the hit that Konami expected. Castlevania III would use branching stages instead (as would Dracula X: Rondo of Blood), as they were more common in action-platformers of the time, and true non-linear play wouldn't return to the series until Castlevania: Symphony of the Night. The other changes, including purchasing weapons and the password system, generally made no later appearance.
What's a new game in a successful series without some marketing tie-ins? Konami of America (and their secondary label to get around Nintendo's licensing rules, Ultra) licensed several of their popular video game titles to Scholastic, Inc. be adapted to youth novelizations. Distributed mostly through their "Book Club" fliers given to students and through sales tours of schools, the Worlds of Power (all of which were written under the pen name F.X. Nine) series was utterly mediocre.
As for the novel, titled Simon's Quest, the less said, the better. Simon Belmont, apparently not capable of dealing with the curse himself, travels to our world to recruit an ordinary boy with an unnatural proclivity for chocolate. The story then meanders through Transylvania, as the duo fend off temptation in the form of chocolate and power-ups, finding pieces of Dracula apparently strewn about the landscape. Nothing is particularly clear about the plot of this novel, least of all the point. The moral of the story is "if you eat chocolate, Dracula will kill you." Or something. Only the weirdest of weirdcore gamers would ever want to read this piece of junk.
Castlevania II was also involved in a minor publicity disaster for Nintendo, over the cover of Nintendo Power #2. The cover showed an armored Simon Belmont holding a whip and a severed head, with various gothickisms in the background. The issue was pulled from stands early when Nintendo got letters from angry moms whose children got nightmares after seeing the image. Nothing else ever came of this.