A Hungarian countess and priestess of the dark goddess Hecate, Erzebet believed she could keep her youth by bathing in the blood of young women, leading her to torture and murder hundreds of innocent girls. But her most vile crimes didn't start until after she died. As a vampire, she continued to use the blood of young women to keep herself young, putting her at odds with Hellboy and especially his adoptive father, Professor Trevor Bruttenholm.
The character of Erzebet is inspired by a real Hungarian countess from the Renaissance era named Elizabeth Bathory, who really did murder over a hundred young women and bathe in their blood, believing it would keep her young. She's second only to Vlad the Impaler as inspiration for the classic nobleman vampire in popular fiction.