A YJA Moment - The Last Great Time War

Though it is listed as but one of the many fronts in the Shadow War, few conflicts have had such a widespread impact on reality as what became known as the Last Great Time War, or simply The Time War.  Though many smaller factions and species would be caught up in its wake, the war itself was centered on two time-traveling empires, the Darleks and the Time Lords.

At some point in history, a race of near-humans from the planet Gallifrey found the Time Stone, which they named the Eye of Harmony.  Using the power of the Eye, they created T.A.R.D.I.S. time which allowed them to travel in the time stream.   To prevent their enemies from attacking them, they moved their entire world out of the space-time continuum into the null zone at the end of history which was known as Vanishing Point.  

Being close to the Eye also changed their physiology, granting them a measure of immortality through what they called Regeneration - After a few centuries of life they would completely rejuvenate themselves and become a new person entirely.  While they shared memories of their past life, their apparent age, appearance, and even gender would change.  This new ability would give each Time Lord millennium of knowledge and experience.

The Time Lords set up the Celestial Intervention Agency as their agents in the universe.  The Agency devoted itself to deal with anomalies or existential threats to existence.  They also worked to prevent other species from getting the ability to breach the time barrier.  Chief amongst these were the Darlek Empire, the most dangerous alien race Gallifrey had ever encountered.

Born on the planet Skaro, the scientist known as Davros made a race of cyborg war machines to end the war on their planet but ended up destroying all life that wasn't their own.  Over the years they expanded their empire across known space, only pausing when significant upgrades required a civil war to weed out the inferior models.  They almost achieved temporal awareness several times if not for the swift action by The Agency.

However, one did not account for The Master.  The rogue Time Lord gave the Darleks the technology they need as part of the pact with the forces of Apokalips.  The Celestial Intervention Agency tried to stop the Darlek Empire but it wasn't able to withstand the might of a time-travel-capable armada.  Only their extreme experience with time travel allowed the Time Lords from being completely overrun.

Raging from the Big Bang to the End of Time, the Time War became the most expansive and costly front in the Shadow War.  A thousand alternate timelines were created as each side attempted to eliminate various factions in the conflict, only to be countered by their opponent. The strain on the time-space continuum was momentous and the Agency feared time itself would unravel but were powerless to stop the War.

In the end, it would take another rogue Time-Lord known as The Doctor to end the Time War once and for all.  Breaking into the Forbidden Vaults on Gallifrey, he stole the weapon known as The Moment and issued an ultimatum for both sides to stop hostilities.  When they refused, he activated the weapon, which erased both Gallifrey and the Darlek War Fleet from existence.

Save for a few TIme Lords and a handful of Darleks, all combatants in the Last Great Time War were destroyed.  The Eye of Harmony seemed immune to its effects but disappeared into the time stream before it could be claimed by The Doctor or any of the survivors.  Without the Time Lords monitoring time, the universe gave birth to dozens of chrononauts - Cable, the original Captain Marvel, Chronos, The Flash, Kang the Conqueror, Per Degaton, Rip Hunter, and the Time Trapper to name but a few.