A bloodline curse is a powerful ritual used to destroy an entire bloodline. It is first mentioned in Changes.
The ritual itself involves the killing a member of a family, causing the death of all the blood relatives of the victim. Since the curse does not spread to children, the death of a family's youngest member is the best way to ensure the destruction of the whole bloodline. The curse also works on vampiric bloodlines, where the "parent" is the vampire who transformed a victim.
The energy required for it is enormous, causing the Red Court to pick Chichén Itzá, a site of power, as the place for the ritual to be performed. No one has used power on this scale in more than a millennium.
The ritual requires the heart to be teared out of the victim's chest and to be ripped to pieces in the process, similarly to the curse that Victor Sells used years before. While Sells didn't have the strength to make the curse spread past the initial target, the Red Court had.
In the series Edit
In Changes, the Red Court attempts to use a bloodline curse on the family of Margaret Angelica Dresden, intending specifically to hit her great-grandfather, Ebenezar McCoy. Acting on the information provided by Donar Vadderung, that he can find his daughter at Chichén Itzá, Harry Dresden is able to use the curse setup to obliterate the whole Red Court by sacrificing Susan Rodriguez on the altar.