Castlevania: Aria of Sorrow
In 1999 AD, Dracula was finally defeated. His castle, the demonic creature of chaos, was sealed away in an eclipse, and Dracula's spirit was finally laid to rest. And yet, the followers of Dracula were never able to let go (like a set of very clingy stalkers). When another eclipse happened 36 years later, Dracula's minions made another attempt to bring Dracula back.
Seemingly unrelated to this plotline, we are introduced to Soma Cruz, an exchange student (from somewhere else, in Japan). While on a visit to the Hakuba Shrine with his friend Mina Hakuba (an inside source!), Soma and Mina are transported to a mysterious castle -- Castle Dracula. There, they are assaulted by monsters, and Soma has to fend them off. In the process, one of the monsters' souls is freed, and Soma absorbs it into himself, stealing a bit of power he can reuse.
A strange man, Genya Arikado, comes to the two students and explains that they are stuck within the demon castle until the mystery is unraveled, and that Soma has to be the one to explore the castle for reasons that would eventually be made clear. During his journey, Soma also encountered Yoko Belnades, a sorceress working for the church, there to ensure Dracula is not, somehow, resurrected. He also encountered J, a mysterious man with no memory of who he was or why he was at the castle... and is later revealed to be Julius Belmont. Julius was inside Dracula's castle when it was sealed in the eclipse, and once he regains his memory, Julius set out to stop Dracula's true resurrection.
The other major player in the castle is Graham Jones, a dark, villainous man who swears that he will be the vessel for the resurrection of Dracula (he even exhibits a number of very Dracula-like powers). Soma had to battle through the castle, defeating the castle's minions, and working to find out if Graham will be the new dark lord...
But the truth is that Soma was the true reincarnation of Dracula (which is why he could collect souls). In an encounter with Julius, Julius sees Dracula's spirit, but also the purity of Soma's soul, and lets Soma live. Soma then goes on to fight Graham, defeating the presumptuous king, and learning the truth about his own existence: he's Dracula, but he doesn't have to be evil. With a final jaunt into the Chaos Realm -- the source of all power for Dracula and his demonic fortress, Soma encountered Chaos itself. In the course of the last, drawn out battle, Chaos was defeated, the castle was permanently sealed, and everyone fled the castle, safe in the thought that Dracula was never going to come back to try and rule the world again, all because he was now a pure soul.
In the game, Soma collects various souls from the enemies he defeats. Some give him attacks (such as throwing fireballs, bones, or javelins), others give him stat bonuses or defensive benefits, and others still give him secondary abilities (such as turning into a bat) to use in exploring the castle. A specific combination of souls (Flame Demon, Giant Bat, and Succubus -- fireball, change into a bat, and heal from the blood of your foes) are required to get the best ending of the game (the defeat of Chaos). Without those souls equipped during the battle against Graham, Soma is not revealed to be the true embodiment of Dracula, and thus doesn't gain access to the Chaos Realm to seal Dracula's power away permanently.
An alternate ending of the game, where Soma loses to Chaos, has Soma becoming the Dark Lord. A similar allusion to this ending is made in Dawn of Sorrow. This ending is, of course, non-canonical.
Unrelated to this ending (at least, in this game), you can also unlock Julius mode. Julius will go about the castle, kicking ass and taking names (the editors of the Inverted Dungeon are big fans of this mode). As with many bonus-character modes in previous games, Julius has no actual bearing on the main plot of the game.
One additional character plays a role in Aria of Sorrow: the merchant Hammer. He's worth mentioning, mostly because he shows up in both games (Aria and Dawn), but aside from selling you various items you may find useful, Hammer doesn't have much bearing on the plot. We at the Inverted Dungeon, though, are currently writing a pilot for Hammer of Justice! that we hope will rectify this slight.