Over the decades, Doctor Who has introduced a complex web of lore, including weapons like the Moment that need a little more explanation than is generally given. When the series was revived by Russell T. Davies in 2005, a critical addition to the Doctor's backstory was that Gallifrey had been destroyed by the Doctor's hand in an attempt to stop the Time War and eliminate the Daleks. However, the question of how the Doctor managed such a massive task was left unanswered until the 50th Anniversary special, "The Day of the Doctor," when viewers finally got a glimpse at the Doctor's darkest day.
In order to prevent the war from destroying the universe, the Doctor made the terrible choice to commit genocide against the Daleks and his own people, a decision which colored the Doctor's perspective during all of modern Doctor Who. The Time War had a huge impact on rebooted Doctor Who as the battle between the Gallifreyans and the Daleks left several planets burning and culminated in the presumed extinction of both species. Although the act was well established, the weapon and its properties still require more explanation to be fully understood.
The Moment Was The Weapon That Destroyed Gallifrey
The Moment is a sentient weapon that could feasibly destroy anything, nicknamed the "Galaxy Eater" by the Time Lords and never used because it judged its . Depending on the canon, with different s appearing in the short story "Pandoric's Box" and the comic stories The Forgotten and Don't Step on the Grass, the device was either created by the ancient Time Lord scientist Roppen or the Doctor himself, in his eighth incarnation. Either way, it was sophisticated enough to develop not only sentience but a conscience that was fundamentally opposed to its own use (similar to the TARDIS's resistance to carrying weapons).
As for the weapon itself, the Moment was among the forbidden weapons in the Omega Arsenal, a collection of weapons that were unconscionable to use (though all had been deployed against the Daleks except for the Moment). The Moment had the ability to destroy seemingly anything in its path, from a planet to a galaxy, with the only limitation being that the person who used it would be punished by the Moment personally. For example, when the Doctor shared that he wished to die, the Moment decided that his punishment would be his survival and the guilt that came with it.
The Time War fundamentally broke the Doctor, which was exploited by the Moment's telepathic abilities. In addition to being sentient, the Moment could read thoughts, see the past and future of its , and take on a holographic form to communicate with the directly. Using the information gained from those conversations and explorations, the Moment could also break the rules of time travel, opening time fissures, breaking time locks, and allowing individuals to interact with their own timelines without causing paradoxes. Between all of these abilities, the Moment was generally able to manipulate its intended s into changing their minds.
How Did The Moment Work In "The Day Of The Doctor?"
In "The Day of the Doctor," the Moment interacted with the War Doctor, played by John Hurt, in order to stop him from completely destroying Gallifrey (thus retconning the foundation of modern Doctor Who). The Moment had actually been mentioned in season 4, episode 18, "The End of Time Part 2," when the Time Lords were looking for a way to escape the Doctor's attack. While their attempts to save Gallifrey failed, the Moment itself succeeded in causing a crisis of conscience in the Doctor.
When the War Doctor seemed certain in his mission, the Moment opened time fissures to show him "the man today will make of you," in the form of the Tenth and Eleventh Doctors. The War Doctor needed to see the men he became, who the Moment dubbed "the man who regrets and the man who forgets." However, that tactic failed, as the War Doctor gradually realized that the guilt of destroying Gallifrey was part of what made the modern Doctors into heroes.
The Moment had one last card to play in the form of true Doctor Who companion, Clara Oswald. The Tenth and Eleventh Doctors were willing to help the War Doctor push the button, sharing the burden of destroying Gallifrey, but Clara pushed for them to the meaning of being the Doctor and the promise that came with it. It was great to see how much faith Clara had in the Doctor as her hopes and beliefs in the Time Lord gave the Doctor the hope he needed and a solution for how to end the war without using activating the device.
Why Did The Moment Look Like Rose?
Bringing Billie Piper back alongside the multiple Doctors for the 50th Anniversary was a move that was really pleasing to see and expanded the lore of the Moment itself. There was definitely a dimension of fan service in the writers' decision to have the interface of the Moment appear in the form of Rose, as it was a callback to her becoming Bad Wolf in the season 1 Doctor Who finale. However, the interface never interacted with David Tennant's Doctor, which would have been a critical part of the fan service, suggesting that the Moment's appearance had more to do with lore than celebrating the show's history.
In “The Day of the Doctor,” the interface clarified that the form she took was specifically Bad Wolf, the omnipotent version of Rose introduced in season 1, episode 13, "The Parting of Ways." By looking into the Time Vortex, Rose gained the power to cast herself through time and leave clues, as well as having the ability to fundamentally change the world around her. As she explained, "I am the Bad Wolf. I create myself," with each connection between Rose, the Doctor, and “Bad Wolf” being deliberate. This detail puts into question whether Rose could be personally responsible for the Moment’s appearance.
It’s even possible that Rose as the Bad Wolf actually was the Moment. The Bad Wolf was tied to the Time War from her earliest creation, and her ability to see "All that is, all that was, all that ever could be" is completely in line with the Moment's actions. While it’s unlikely that the writers intended this originally, it is a piece of retroactive lore that would justify in-world why the Moment looked like Rose. Altogether, the Moment was a critical device on Doctor Who that may be seen again the next time the fate of the Doctor's soul is at stake.