Celia Fortner is a mystical priestess who serves as the primary antagonist of the Castlevania game Dawn of Sorrow. Since Count Dracula was destroyed for good in 1999, Celia is looking for someone able to succeed him as the new King of the Night. She recruits Dmitrii Blinov and Dario Bossi as candidates and meddles with Dracula's reincarnation, the primary protagonist Soma Cruz.
Celia Fortner is a young and immensely powerful mage-priestess, expert in spellcasting, rituals, Magic Seals, and demon summoning, being able to create her own Demon Castle and to control hordes of monsters and evil forces.
She is the founder and leader of a mysterious religious cult known as the "With Light", who believes that good cannot exist without evil and that the King of the Night is necessary for the balance of the world. (Her view is stated to be true by the protagonists, but Dracula's son Alucard explains that the balance will eventually restore itself on its own.) As such, she set out for people who would be able to inherit Dracula's throne.
While Celia is the most ambiguous antagonist of the series, being more neutral than anything, it must be noted that she is extremely self-righteous, inflexible and ruthless to the point of fanaticism, being willing to do anything to accomplish her goal without bothering with moral standards.
Her official background states that Dracula's demise weakened the magic forces in the world. As such, she also seeks the King of the Night's Second Coming out of interest, to maintain her influence and to gain more from the King of the Night. It can be assumed that she sees herself as a guide meant to lead the people of the world, whether they like it or not.
Celia is observant and calculating, setting the stage and making her moves following the events. She is very manipulative and intelligent, being adept in finding people’s triggers to influence their actions. Yet, she is too conceited to realize that her pawns have agendas of their own that can (and will) clash with her plans, leaving herself open for backstabbing.
Celia Fortner recruited Dario and Dmitrii, who like Graham Jones before them were born the exact day Dracula was destroyed and inherited some of his Dark Powers. She also heard of Soma Cruz, Dracula reincarnated and redeemed, who defeated Graham and Chaos, the very essence of Evil in 2035. One year later, she opens a Portal towards the Dark World and creates a replica of the Castlevania (which had been sealed in a solar eclipse) around it, and populates it with hordes of demons and monsters who used to serve the King of the Night. Deeming Soma unworthy, she seeks to have him killed and replaced by her two candidates.
In the beginning of the game, she appears before Soma (who lost his powers) and his girlfriend Mina Hakuba, and summons skeletons to kill them, but Alucard, Dracula's son, appears under his human aspect. She casts a barrier spell to keep him at bay but he gives Soma a weapon, and teleports away when the skeletons are destroyed.
Celia later lures Soma within her Dark Castle and plans to make her three candidates fight to the death to see who would prevail. She teleports Dario away when the latter is about to be defeated by Soma, and fuses him with the Fire Demon Aguni to increase his powers before setting up a rematch. If Soma kills both Dmitrii and Dario, Celia's plan will be trashed and she will flee her Castle, never to be seen again.
If Soma defeats Aguni instead of Dario, however, she attempts to make Soma fall into Darkness like Mathias Cronqvist before him. She uses a doppelganger shape-shifted as Mina to stage a trap. When Soma confronts her, she pretends to hold Mina captive and to kill her before his eyes, so that he would be overcome with hatred and fall into Evil.
If Soma watches this without wearing Mina's pendant it triggers the bad ending, in which he becomes the King of the Night and kills Celia with a surge of Dark Energy stating "You annoy me and that's all the reason I need." This triggers the Julius Mode in which the player incarnates Julius Belmont, the man who destroyed Dracula, to fight against Dark Lord Soma, restarting the never-ending battle between Good and Evil.
If Soma watches this with Mina's pendant, Alucard barges in to reveal the subterfuge, saving Soma from corruption. But it now appears that Dmitrii let his soul be absorbed by Soma after his defeat to copy his powers from the inside, and possesses the doppelganger to regain his physical body.
Celia then recognizes Dmitrii as the worthiest successor and teleports with him towards the depths of the Castle, but she outlived her usefulness in his eyes, so he sacrifices her to summon countless demons with whom he overwhelms Alucard. He later sets her corpse on display until Soma barges in and destroys as a demonstration of his newfound power.