Intended to be released in 2000 or 2001 exclusively on the ill-fated Sega Dreamcast console, this even more ill-fated entry in the Castlevania franchise was meant to take place in the year 1666, when the power of evil was at its strongest. A European countess had fallen in love with the mystique of the evil Vampire Lord Dracula, and used her magical knowledge to once again bring the Prince of Darkness back into the world of the living. However, during this period of time, the power of the Belmont clan was also at its lowest, and there were no heroes capable of standing against Dracula's evil.
To counter this, the forces of Heaven took a rare stand of direct interference, and forcefully summoned to this point in time two mighty warriors of the Belmont family from elsewhere in history; one from the past, and one from the future, the two playable protagonists of Castlevania: Resurrection.
From the 15th century they took Sonia Belmont, protagonist of Castlevania: Legends, who was the very first member of the family to wage war against Dracula's evil (at least until her game was later unfortunately and inexplicably removed from the official canon), now bringing her whip and fearless nature to bear once again, more than two hundred years after her last adventure!
From the 19th century comes Victor Belmont, a new character who was described as a wandering gambler and mercenary who had rejected the Belmont family destiny in his own time, running away from home upon learning of the great responsibilities that his family carried. However, while living a life on the road, he'd still learned the skills of martial combat, studying "not just the art, but the science of warfare", and become a powerful warrior and soldier of fortune. Having turned his back on his duties in the 1800s, Victor, who bore a strange resemblance to then-popular Canadian comedian Tom Green, was given this opportunity to save the world in the 17th century and prove himself worthy of his bloodline.
Both of these heroes had been plucked from their lives against their will and pitted against Dracula and the Countess in an era that was not their own, and only by successfully defeating the Lord of Vampires and saving the world could Sonia and Victor be returned to their own times.
It is unknown how exactly the two protagonists would have worked, whether the player would pick one and play all the way through the game, or if you had to beat the game as one to unlock the other, or even if there was a single linear progression to switch back and forth between them (in the manner of the much, much later Transformers: Fall of Cybertron's single campaign, which contains both Autobot and Decepticon characters). What is known is that the game was not terribly far into development; although screenshots, concept art, and a playable demo existed at some point, the art related to the Countess who brings Dracula back to life didn't even give her a name, and only referred to her by the ridiculous title of "Female Dracula". Additionally, the game would have abandoned the free roaming of popular games in the franchise like Castlevania: Symphony of the Night, or even the large, wide-open areas seen in Castlevania 64 in favour of a much tighter, more linear A-to-B style of gameplay that focuses on the combat. With this in mind, it's probably for the best that the game was ultimately cancelled in March 2000, apparently because Konami had no confidence in the Dreamcast platform itself. Coincidentally, this cancellation was made official on the very same day that Sony officially announced the Playstation 2.
Sonia Belmont would never appear in a Castlevania game again, and as mentioned before, Castlevania: Legends was removed from the official canon, with Leon Belmont eventually taking Sonia's place as "the first Belmont" with the release of Castlevania: Lament of Innocence in 2003. However, every other game that was declared non-canon was eventually brought back into canon, so it remains possible, however unlikely, that she may return. Victor Belmont, on the other hand, disappeared with the cancellation of Resurrection, and was never seen or referred to in any Castlevania game until 2014's Castlevania: Lords of Shadow 2, which took place in the Lords of Shadow reboot universe. As with other characters who 'reappeared' in the Lords of Shadow setting, this Victor Belmont was nothing like the original, and was instead a dedicated warrior who embraced his destiny, the active member of the Belmont clan in the year 2047, and the leader of the monster-hunting organization called the Brotherhood of Light.
In 2021, a copy of the behind-closed-doors E3 1999 demo was discovered.