The "sacrificial maiden" (unofficial name) is a recurrent concept in the Castlevania series. It involves a young woman who has been abducted by a sect known as the Followers of Darkness in order to be offered as a sacrifice and bring with this the Dark Lord, Count Dracula, back to life.
The first maiden appeared in the opening cutscene of Castlevania: Rondo of Blood, although her role in that game or her background has never been explained. She was an unknown girl who was presumably abducted from a nearby village and brought to an abandoned church by the dark priest Shaft and his followers; she was then sacrificed in a black mass in order to revive their Dark Lord, Count Dracula.
The second time one of these maidens appeared in a game took place in Castlevania: Legacy of Darkness, in Cornell's storyline. In this case, the black mass was performed by more prominent attendants, such as the foul witch Actrise, her servant Gilles de Rais, and even Death himself.
In other occasions, the leader of the ceremony can be seen holding a (presumably) human heart on their hand. Although not stated, it's possible that this heart could have belonged to one of these maidens, being hacked out of her body prior to the beginning of the ritual.