|Source: Dresden Files|
The Bloodline curse is a powerful ritual used to kill an entire bloodline.
The ritual itself involves killing a member of a family, which also brings death to the victim's siblings and parents, then their siblings and parents, etc. until no one is left. This is why killing the youngest member of a family is the best way to make sure the whole bloodline dies, as it doesn't seem to spread to the children.
The bloodline curse also works on vampiric bloodlines, where the "parent" is the vampire who transformed a victim