"Destruction is everywhere. All things die. I am the only destiny of everything. My voice is your fear." - The Enemy
The Enemy of All Things is a primordial, immortal sentient force of pure malignance that has no other goal but to devour all matter, vapour and life in the Universe. He attempted to use the TARDIS and his own star prison to gain god-like power. He was eventually killed by being thrown into a white hole and burned out of existence.
He did, however return and became unspeakabley powerful after a localised event collapse returned him and the entity, Little Horn into life again and they then battled for the universe, destroying billions of galaxies in the onslaught. He was eventually destroyed again along with Little Horn by the Doctor.
During the Dark Times, the entity that would become the Enemy went from galaxy to galaxy, destroying everything until only the void of space remained. The Time Lords imprisoned the Enemy within a star that they created by reversing the process that they used to create the Eye of Harmony.
The black star, known as Yamagel, became the centre of an entire star system, with the first planet in its alignment, Shaak Taar, being its 'jailer', should the Enemy ever find a way of escaping before the star collapses and destroys it.
After the decimation of the Time Lords, making Yamagel's binding strength weak, the Enemy managed to only slightly escape - but at the cost of its colossal form, fashioning a replacement using the basic template of its imprisoners, the Time Lords and becoming the black winged shade it is so thought of to be.
Needing a 'solid' form to sustain itself, or else it would fade into dust, the Enemy travelled to Shaak Taar at the time of their Shaar Tuk (their most celebrated festival). It used the form of a young Shaak Taar soldier, known as Shaak Krue, seeing him as the perfect specimin to use when destroying the Universe. The Enemy masqueraded as the planet's deity, Shaak Turri and was using Krue as a 'vessel' to send Shaak Taar into salvation against Yam Tukaa (the Enemy) (DW:Sand Devil).
In reality, the Enemy was going to have the Shaak Taarians send Yamagel into the planet's core, destroying the planet and giving the Enemy enough 'destructive energy' to gain its full power once more.
Defeat and death
Unfortunately, the Doctor, Martha Jones and Kale Phoenix were also on the planet and learned quickly of the Enemy's true identity. Aided by Shaak Zuin, Krue's friend, they prevented the disaster, and revealed to the entire population that Krue's mind had been destroyed and that Yam Tukaa is the real entity within him.
The Enemy attempted to hijack the TARDIS and have it pull it into Yamagel in order to gain so much energy that it could become a god-like being and instantly wipe out the Universe and even move on to other realities.
The Doctor tricked the Enemy into sending the TARDIS (alone), to a white hole and the Enemy was thrown into the heart of it by the 'angered' TARDIS. The 'creative energy' of the white hole reacted violently to the Enemy's negative form and the Enemy was 'cancelled out' by the white hole, being eliminated from existence forever.
Following the events of his death, several attempts to disrupt reality were made which caused the memory of the Enemy and another entity to bleed out into existence and spawned two 'houses', both devoted to the resurrection and dominance of their respective 'gods'. The Enemy came in the form of 'the Tiger', an all-devouring monster that brings death and dust.
Opening the 'doorway'
In 2020, a localised event collapse caused in another universe brought the Enemy into existence again, more powerful than ever. He landed on Earth, in the form of a human and was about to kill the Doctor and then devour the universe when another human suddenly fell from the sky.
This new entity, known to be the most feared being in all reality, Little Horn, desired the Doctor and the universe for himself to destroy and reshape. He demanded that the Enemy step aside. The Enemy refused, and this sparked the battle that the two Houses had prophesised - the Dragon against the Tiger. However, whoever would win this battle, will destroy everything.
Fighting Little Horn
The Enemy was said to be invisible before it was imprisoned and was only seen coming when matter began to rapidly decay and vanish. Once it was imprisoned in Yamagel, the form of the Enemy was shown as an ever-changing black mass within an orange star. When it broke out, the Enemy appeared as a black shade-like Shaak Taar/Time Lord with black wings.
When the Enemy returned from death, he was thought to be the only one and with his increased power, assumed human form and destroy the Doctor and Earth slowly. His appearance is that of a young teenage man, dressed all in black with wild hair and grey eyes. He retains his black wings and his eyes will become red if expressing great emotion, like rage or lust.
In his 'Tiger' form, the Enemy appears as a giant black cat that seems to be composed of both shadow and fire. In this form, he can grow from normal tiger size but his 'default size' is so big that even his eye is bigger than the Earth, equally as large as Little Horn's 'Dragon' form. The Enemy only became this size when he saw that Little Horn had arrived.
The Tiger form also mutated when he grew in size and after being attacked so much by Little Horn's Dragon form. It became more alien-looking and eventually looked almost nothing like a tiger at all, just a bestial mass of shadows and super-heated plasma.