Family of Blood

The Family of Blood were a family of hunters who hunted the Tenth Doctor in order to take his near-immortality.

Biology
The Family of Blood were green, gaseous predators 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, and, if they went without possessing anyone, they would perish in just three months, "like mayflies." (TV: Human Nature)

The members that were chasing the Doctor consisted of a father (Father of Mine), mother (Mother of Mine), son (Son of Mine) and daughter (Daughter of Mine). While in human form, the Family could telepathically communicate with each other from a distance. They always referred to each other by their relationship followed by "of Mine", i.e. Father (or Husband) of Mine, Mother (or Wife) of Mine, Son (or Brother) of Mine and Daughter (or Sister) of Mine. Father of Mine referred to Mother of Mine as "Wife of Mine" when reporting that he had found the Tenth Doctor's TARDIS, indicating that the two were in fact married (or their equivalent of such).

They also had an incredible sense of smell, tracking the Doctor wherever the TARDIS took him by his scent alone. Unfortunately, whenever they possessed someone, their victims' minds and memories were mostly obliterated; the remaining knowledge was barely sufficient for the Family to successfully pose as their victims without arousing suspicion. (TV: Human Nature / The Family of Blood)

Technology
The Family utilised a spacecraft, that held their 'essence' within it prior to them assuming human form. The interior contained green lights, pillars and paddle-like switches (somewhat similar to the Doctor's TARDIS) and seaweed-like plants dangling from the ceiling. The spacecraft had a considerable arsenal of technology — the most notable of which was a Vortex Manipulator (stolen from the Time Agents) that allowed the craft to travel through time. It could also project force fields, render itself invisible and had bomb-shooting cannons.

On hand, the Family of Blood used energy gun that fired green energy that disintegrated humanoid tissue — the weapons apparently consist of a living creature within a shell that shot the beam when pierced by the trigger. They were somewhat difficult for humanoids to hold. The Family also had scarecrow-like soldiers created by Son of Mine and placed in 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. (TV: Human Nature / The Family of Blood)

History
The Family encountered the Tenth Doctor and Martha Jones, and chased them throughout time and space with a stolen Time Agent vortex manipulator, hoping to steal from the Doctor some means of ensuring that they would live a longer lifespan. According to Son of Mine, on the Family's travels through time and space in search of the Doctor, they saw many things including the First World War.

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 eventually came there and landed nearby, and activated animated scarecrows to round up humans to possess as bodies. (TV: 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 Tim Latimer. He fooled the Family by pretending to still be John Smith and using an olfactory misdirection (said to be like "ventriloquism of the nose") to trick them into thinking that he was still human. Moments later, an act devised to look like a clumsy accident by "John Smith" resulted in the Family's ship overheating and exploding. The Doctor subsequently punished the Family by:


 * Wrapping Father of Mine in unbreakable chains forged from a dwarf star alloy and imprisoning him in an underground chamber.
 * Sending Mother of Mine into the event horizon of a collapsing galaxy (black hole), trapping her there for all eternity.
 * Trapping Daughter of Mine within every mirror in existence, where the Doctor would still visit her once a year (she was said to be the strange movement seen out of the corner of one's eye when looking in a reflective surface).
 * Imprisoning Son of Mine, now locked in time, in the body of a scarecrow to watch over the fields of England as their protector.

"We wanted to live forever. So the Doctor made sure that we did."

- Son of Mine on the Family's fates.

In a sense, the Family's quest for immortality was won, although not in the manner they wished. They also discovered from this why the Doctor decided to flee rather than fight them — he was showing them mercy, by allowing them the chance to live out their lives then die, rather than killing them or dooming them with immortality. (TV: The Family of Blood)

The Visual Dictionary
The non-narrative source Doctor Who: The Visual Dictionary gave further information about the Family of Blood.

When taking over a human body, the human's memories are destroyed in the progress, although it is still possible for traces of it to remain intact.

This is not even enough to live on without the Family piquing suspicion.

Their telepathic communication means that when in very high trauma, others can feel this pain.

Laser guns are their weapon of choice.

Even when possessing humans, their sense of smell is just as good as gaseous form.

Their natural form is ball-shaped gas.

They seek the life of a Time Lord because of their notably short three-month-long lifespan.

When taking over human bodies, the Family's gaseous form enters them through the eyes.