Satan

Satan (also known as Lucifer, and sometimes as The Devil) is a demon and the ruler of Hell, said to have been a 'dark lord'.

In the Castlevania series
The original Japanese instruction booklet for Castlevania III: Dracula's Curse (Akumajō Densetsu) states that Dracula –apart from selling his soul to the Devil– also forced his son, Alucard, into the contract in order to obtain even more power. However, this part of the series' lore was removed in Symphony of the Night with the inclusion of Lisa into the storyline, making Alucard a dhampir from birth and removing his initial status as a purebred vampire.

In 1792, cultists call upon the power of Satan, the Devil, in order to resurrect Dracula again.

In 1844, Cornell, or in 1852, Reinhardt Schneider and Carrie Fernandez, agree to a contract with Satan not to spend over 30,000 gold. If they spend more than this, Renon will appear before them and reveal the fine print in his contract, revealing his true identity, and coming to enforce the contract with them to make their souls his. At which point he transforms into his true demonic form to fight the characters.

Satan is also occasionally mentioned in items in other games of the series, such as Satan's Ring, which is said Satan himself once wore (note this only appears in English localizations; the 'Dark Lord' mentioned is generally believed to actually be Dracula himself).

Lucifer's Sword is an artifact found in Castlevania: Lament of Innocence in 1094.

Controversies
There's some discrepancy regarding the existence of Satan/The Devil in the Castlevania series. While some games do mention his existence along their documentations, other games outright ignore this fact and/or even present alternate variations to certain concepts such as the Devil and/or Hell.

As the series progressed, the concept of a superior being who controls all evil in the world (and perhaps even in the entire Universe) has befallen solely on what's known as the "Dark Lord" (or the "Prince of Darkness"), with the centuries-old vampire, Count Dracula, generally bearing this title, whereas in most folkloric traditions –outside the series– Dracula is just an emissary of evil, albeit very powerful indeed, probably only second to the Devil himself.

Likewise, the concept of Hell in the series has had its own particular interpretation more than once and in more palpable forms. Alternate realms parallel to this world, such as the Chaotic Realm and The Abyss, could be considered interpretations of Hell. Not only that, but the force of Evil itself has had its own interpretation in the series as well, such as is the case of Chaos, which, as its name implies, is the origin of all chaotic things and the source of Dracula's "evil" powers, as he waits for the next time he is revived. These powers –in the more traditional knowledge overall– are generally alleged to the Devil instead; however, mentions of him have notably diminished as the series progressed, with more recent titles barely acknowledging him, or if they do, at least not with the same level of relevance.

It's worth mentioning the role Satan plays in the Lords of Shadow series, though; a reboot of the Castlevania series that attempted to present a deeper background to previously established concepts and richer development to its characters. In this series, Satan plays a role much more faithful to traditional religious history, where he is indeed an angel that rebelled against God and who plans to defeat Him and usurp His throne as ruler of Heaven. For this goal, he sets along an intricate plan that involves a series of events where characters are played upon as if they were chess pieces, with the ultimate goal of getting his hands on a relic (relics, in fact) of (apparent) ultimate power, known as the God Mask. Near the end of this conflict, and after the Brotherhood hero, Gabriel Belmont, defeated his deceitful former "friend" and mentor, Zobek, Satan himself materialized to him in apparent flesh and bone and revealed the truth and his intentions - offering Gabriel to become his paladin in the war that was about to come. However, Gabriel –disappointed– refused his offer and bested him in combat, thwarting Satan's plans and sending him back to the Underworld.

This was not the last that would be heard about Satan, however, as his three Acolytes made sure to still try to make him to achieve his plans of total domination; although, in the end, all three failed and met their demise, with one even being slaughtered by Satan himself.

Trivia

 * Sons of Satan is a musical theme that plays in Castlevania II: Belmont's Revenge during the final battle against Dracula.
 * Some of the titles of music themes from the other translated games include the name "Satan" (although it's sometimes translated as 'Devil' as well) (see Konami Game Music: Classic Castlevania).
 * The gamebook The Legend of Satanic Castle: The Vampire Hunters contains a reference to Satan in its title.
 * The word "Castlevania" was created by Emil Heidkamp, Konami of America's senior vice president at the time, who believed the game's Japanese name was translated as "Dracula's Satanic Castle", and therefore he chose to change it out of potential religious sensibilities.