The most famous weapon of the Castlevania series, the Vampire Killer is a whip that was originally created by Rinaldo Gandolfi through alchemy in Castlevania: Lament of Innocence. Rinaldo had made the whip intending to use it to defeat the vampire Walter Bernhard. However, the whip proved to be ineffective. Once Leon Belmont arrived at his cottage, Rinaldo gave the whip to him, hoping that he would find a way to defeat Walter. It was only after Leon's betrothed, Sara Trantoul, sacrificed her soul to make the whip complete that it would actually gain the power to destroy vampires. It was from this point forward that the whip became known as the Vampire Killer.
The whip has the ability to destroy anything that is related to the vampires or the night in general. Even Death, who is a servant of Dracula and not an actual vampire, can be harmed by it because he is associated with a vampire. The whip also seems to have a will of its own, able to feel emotion just like a person would. This is probably due to the fact that it contains the soul of Sara Trantoul. At one point in "Lament of Innocence," Leon says that he actually feels rage coming from the whip in the presence of a vampire.
In the early games of the series the Vampire Killer would begin as a leather whip and could then be upgraded to a short morning star, followed by a long morning star, after finding power-ups for it. Later games in the series would see the whip not changing at all.
It also seems that the whip's power can only be fully utilized by a member of the Belmont family, as is the case in Castlevania: Portrait of Ruin. In this game, the character Johnathan Morris wields the Vampire Killer but is unable to use its full power because he is not a direct descendant of the Belmont bloodline. A non-Belmont wielding the whip can also be dangerous, as it drains their strength with prolonged use.
The Dagger is one of the most common sub-weapons and is found in almost every Castlevania game. It is typically a low damage weapon that is thrown in a straight line and costs only one heart to use. In later games in the series, such as Lament of Innocence and Harmony of Dissonance, the player can use magic to increase the power of the Dagger and other sub-weapons to perform various other attacks, such as multiple daggers being thrown instead of just one.
In Castlevania II: Simon's Quest there are actually three different forms of the Dagger, ranging from a weak short-range weapon to a powerful long-range one.