Long Story short: Crusader Christof Romuald gets wounded in the intro and the nun Anezka is nursing him day and night, which saves his life. To thank the convent and it's Bishop for saving his life he fights undeads for them, only to get bitten and turned into a vampire himself. But she doesn't reject him, and instead gives hope for them to rejoin, but Christof lost his hope already at that point.
Later, you find out that she went by herself to vampire clans, where her purity is so radiant she almost burns them. She gets turned nontheless by a Methusalah (a very old and powerful vampire), but turns out she does it on purpose so she can reunite with his love while also stalling her sire's plan to end the world.
Centuries later in the modern world, Anezka reawakens Christof who got trapped under the rubble of a collapsing building in the medieval times. He slowly finds out about what is about to happen and that Anezka will be there too.
Now depending on how you played, you can become happy together, but also be forced to kill her, but either way: Who else can claim pursuing your only true love for an entire millenia and get a happy ending?