Bad Wolf (entity)

After looking into the Heart of the TARDIS, the Time Vortex itself filled Rose Tyler, who then temporarily became the Bad Wolf entity.

Rose Tyler as the Bad Wolf
Newly empowered, she piloted the TARDIS back to the Ninth Doctor. A nimbus of light surrounded her and shone through her eyes. Rose's natural London accent vanished. She also possessed the destructive and creative powers of a goddess, enabling her to destroy an entire Dalek fleet and have an enormous influence over time and space.

While possessed of these gifts, the entity set out to spread the Bad Wolf meme through many worlds and times. In this way, the Ninth Doctor and Rose's past selves would eventually follow the clues that would enable them to defeat the Daleks, creating an ontological paradox in the process. The Bad Wolf destroyed the Dalek fleet and all the Daleks on Satellite Five, including the Dalek Emperor. She also altered reality to bring back Jack Harkness from death. (TV: The Parting of the Ways)

Jack would later discover he could apparently no longer die by any means, and it would take him nearly a century and a half of subjective time before he finally learned that she was responsible. (TV: Utopia) As the instigator of Jack's immortality, Bad Wolf was therefore indirectly responsible for the event known as Miracle Day on Earth. (TV: The New World, et al)

Rose was unable to hold the power permanently without burning up, and the Doctor restored Rose's human self by absorbing the Vortex energy into his own body, a process that would prove destructive to his cellular structure and so prompt the Doctor's next regeneration. (TV: The Parting of the Ways) Rose subsequently lost most of her memory of her time as the Bad Wolf, saying that the knowledge had been locked like it was "forbidden", and that trying to repeat it could cause the universe to blow up. (TV: The Christmas Invasion)

It has yet to be explained how Rose, or the entity, arrived at the name "Bad Wolf".

Abilities
The Bad Wolf entity was incredibly powerful and could destroy an entire Dalek fleet by dividing all the atoms of the ships and the Daleks themselves. (TV: The Parting of the Ways) It also had the ability to resurrect already dead beings and make them immortal. (TV: The Parting of the Ways, Utopia) It could also form a force field to defend itself from Dalek weapons and spread the words "Bad Wolf" to various points in time and space so that Rose and the Doctor would encounter the words which eventually led Rose to becoming the Bad Wolf in the first place. The Bad Wolf stated "I create myself" which was a paradox. (TV: The Parting of the Ways)

Later references

 * Upon her meeting with the werewolf host, that entity noted that Rose also had "something of the wolf" in her. (TV: Tooth and Claw)
 * After being captured by Torchwood, the Doctor pretended Jackie Tyler was his companion Rose. He claimed that she had stared into the heart of the time vortex and aged fifty-seven years. She replied "I'm forty!" (TV: Army of Ghosts)
 * While on the Valiant, commented to the Doctor how he "once had companions who could absorb the time vortex". (TV: The Sound of Drums/ Last of the Time Lords)
 * When Rose and the Doctor say good-bye, they meet on a beach in Norway called Dårlig Ulv Stranden which Rose translates as "Bad Wolf Bay." (TV: Doomsday)
 * Rose whispered the words "Bad Wolf" to a dying Donna in an alternate universe. After Donna remembered these words in the main universe, all the banners and signs in the Shan Shen marketplace to read "BAD WOLF", including the sign on the TARDIS. (TV: Turn Left) This warned the Doctor and Donna Noble of the impending danger of Davros and the Daleks. (TV: The Stolen Earth)
 * Also, all records of the companions of the Doctor that Torchwood had on file were reportedly destroyed or corrupted in some way by a virus known as "Bad Wolf virus". (TV: Love & Monsters)
 * When the consciousness of the Moment takes form, she selects the form of Rose Tyler as the Bad Wolf entity. When the War Doctor comments that he could kiss her for giving him a way out of the dual-genocide he's prepared to commit, she remarks, "that's definitely gonna happen." Both the 10th and 11th doctors are visibly disturbed to hear their past self mention the Bad Wolf, as she is only visible and audible to the War Doctor. Due to the timelines rearranging themselves, the War Doctor (and his immediate successor) retain no memory of Bad Wolf. (TV: The Day of the Doctor)