"No, no, in this form, I'm called...Bad Wolf. Are you afraid of the big bad wolf, Doctor?"
- The Moment in the form of Rose Tyler/Bad Wolf.
Bad Wolf was an "easter egg" / recurring theme/motif in the first season of the Doctor Who revival in 2005/2006. The ninth Doctor (in his only season) and his companion, Rose Tyler, kept running into the words "Bad Wolf" wherever they went in Time and Space.
At first, it seemed it was all the places where the Doctor was going, but in the end, it was revealed to be Rose. During the final episode of the season, the Doctor has to face off alone against the enemy he thought he had destroyed, the Daleks. So to protect Rose, he sent her to the TARDIS and had it take her home to 2006.
Rose, not wanting to leave the Doctor behind and not knowing how to operate the Doctor's TARDIS (his time machine, come back on T-day for that!) forces the console open and accidentally absorbs the heart of the TARDIS. Which is essentially, a contained black hole. She takes the TARDIS back to the future where the Doctor is about to be killed by the Daleks (again, come back on D-day for those guys) and... well it is spoilery if you have not seen it, but it is a great scene.
What is significant here is Bad Wolf/Rose's line "the Time War ends." When we next see the Bad Wolf in the 50th Anniversary episode it/she is the interface for a weapon known as The Moment. A weapon that the War Doctor (the one before the Ninth) was going to use to end the Time War.
Time travel, especially in Doctor Who, is never a straight line. It is, as the Tenth Doctor would say a "big ball of wibbly wobbly... time-y wimey... stuff."
What is great about Bad Wolf is how the Companions of Doctor Who would come to be viewed by the Doctor in this new refresh of the series. This would give us such companions as Martha (the girl who walked the Earth), Donna (the most important woman in the universe), Amy (the girl who waited) and Clara (the Impossible Girl). And, of course, River Song. But she gets her own day. And in 2006 after dealing with the toxic way Whedon treated his characters and actors this was a breath of fresh air. EVERYONE thought Russel T. Davies was going to kill off Rose, but no. He did the opposite.
The Bad Wolf would make other appearances, but never like that first season. Likely good too, the times she has shown up again have usually meant the end of the Universe.
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