Family of Blood

The Family of Blood was a group of hunters who followed the Doctor in order to take his near-immortality.

Biology
The Family of Blood were green, gaseous entities that could inhabit and control Human bodies. Before taking human form, the members of the Family of Blood were kept in small, spherical receptacles. They had very short-lifespans, lasting only 3 months. (DW: Human Nature)

The members that were chasing the Doctor were made of a mother, father, son and daughter. While in Human form the Family could telepathically communicate together from a distance. They also had incredible sense of smell, tracking The Doctor where ever the TARDIS took him with his scent. They also had access to the Host's memory, though they had some difficulty pretending to be Humans. (DW: Human Nature / The Family of Blood)

Technology
The Family utilised a spacecraft, that held their 'essence' within it prior to them assuming human form. It had a variety of paddle like switches and other devices. The spacecraft could be rendered invisible.

The Family of Blood used energy weapons that fired a green energy that disintegrated humanoid tissue. They were somewhat difficult for humanoids to hold.

The Family also had scarecrow-like soldiers created by the son and placed into various fields. They were composed of straw, their molecules animated via a scientific process. Even after being shot down, they could still reanimate by the command of the Family. (DW: Human Nature / The Family of Blood)

History
"He never raised his voice. That was the worst thing - the fury of a Time Lord - and then we discovered why. Why this Doctor, who had fought with gods and demons, why he had run away from us and hidden. He was being kind."

- Son of Mine.

The Family encountered the Doctor and Martha Jones in 2007 and chased them throughout time and space with a stolen Time Agent vortex manipulator in order to steal from the Doctor some means of ensuring that they would live a longer lifespan.

The Doctor was forced to turn himself into a Human using the Chameleon Arch in his TARDIS in order to escape them. The Doctor changed himself into John Smith, a teacher at a boy's school in England in 1913. The Family, landing nearby, activated animated scarecrows to round up humans to use as bodies. (DW: Human Nature)

The Family's quests for immortality ended when the Doctor was returned to his Time Lord state with the help of Martha, school nurse Joan Redfern and Timothy Latimer. He fooled the Family with an olfactory misdirection and destroyed their spaceship by moving several switches in an act devised to look like an accident, and clumsiness on the Doctor's part. The Doctor then punished the family by:


 * Trapping the father in an unbreakable chain forged from dwarf star alloy.
 * Sending the mother into the event horizon of a collapsing galaxy, trapping her there for eternity.
 * Trapping the daughter within every mirror, the Doctor would still visit her once a year. (She is said to be the strange movement you see out of the corner of your eye when looking in a reflective surface).
 * Trapping the son, now locked in time, in the body of a scarecrow to watch over the fields as their protector for all eternity.

In a way, their quest for immortality was won, although not in the way they would have liked. It was also revealed why the Doctor decided to flee rather then fight the Family of Blood - he was showing them mercy, by allowing them the chance to them to live out their lives rather than killing them. (DW: The Family of Blood)