The Family of Blood are the spiritual descendants of the villains from the novel the two parter they appeared in was based on, but they're different enough that the Family are their own beast, and an interesting set of villains at that. Sadistic and ambitious for immortality, they were able to provide a challenge for the Doctor while masquerading in his human form. The Family have an interesting mythos behind them. They are intially green gas clouds with the ability to possess other life forms, but they have really short life spans - the Doctor compared them to mayflies in that respect. However, they're able to animate inanimate objects such as scarecrows to be their soldiers, and their ship and weaponry is another visually interesting piece of design. However, Harry Lloyd's performance is a huge part of why they manage to be so effective. He's scary and threatening every time he's on screen, creating the sense of an intelligent but theatrical psychopath who won't let anything stand in the way of his achieving immortality. Of course, the other members of the family also put in good work, making the entire group of aliens menacing.
