Disclaimer: I don't own anything worth owning let alone something as profitable as Buffy or Who verse.
A battle in a time war is not something that can really be described or explained. For as any time sensitive can tell you time does not necessarily come from a strict cause and effect and it is not in any way shape or form liner. It only appears that way to non time sensitive species. So, a battle where both side can travel back in time and counter their moves quite quickly becomes a ridiculous and complicated affair that very few can ever really understand. But there are those few insanely brilliant persons that don't just understand it but excel at it one such person was a Time Lords who was named the Slayer in the last great time war.
One of the myriad of mysteries that the time war created is the disappearance of the Slayer right there at the end. Many wondered in those last few eternities if the war might have been won if the slayer had not disappeared. But that is question that cannot be answered for the time war is locked away and the Slayer is nowhere to be found.
So what really did happen to the slayer. While the Time Lords of Gallifrey will never know, some do know and below you will a find the tale of the Lord Slayer.
The Lord Slayer was the best fighter on any side of the Time Lords, but even the best lose sometimes. And that is precisely what happened the slayer was hit though two paradoxes and five rifts to come crashing down on a planet that was a dumping ground for many low level monsters created in the timewar. All sorts of vile creatures whom the natives called demons.
The natives had developed a way to take the essence of some demons and though physic manipulations bind them to their warriors who could then protect them from the demon hordes. But finding a demon that this could be done to was not easy and even harder was reducing it to a state where they could do such a thing. So, when the happened upon the slayer they took advantage of this and did the spell to extract it's essence. That part went perfectly as expected, but when they tried to put the essence in one of their young warriors it rejected him. They pondered this and decided to try again but again it rejected it. It was then they were remained by those that found the demon that the demon had a appeared to be feminine. SO, a young girl of the clan was brought before the elders and they tried again, this time they essence went into the vessel. But again the results were not what they were used to. Normal the warriors would feel empowered and they would grow tall and become muscled as no man could. But the girl did not do any of these things for first she passed out and here appearance changed not. The first change they could detect came when she awakened. When the girl awake she was wild and no man not even the great champions could hold he back she moved with grace and had the strength of ten men, but most important of all she was feral and fled the village.
And so she lived in the wilderness and would hunt and destroy the demons so the elders of the village counted the imbuing a success. It was some years later when the girl was killed by overwhelming number of demons that they found out he true depth of what they had done. For in the very same moment the girl had died another girl of the village awake from a nightmare to feel strong and the urge to destroy the demons that she could feel approaching the village. She leapt from her resting place and screamed for the villagers to awake. A scream in the night is not a thing the only village on earth could ignore so they leapt from their rest and beheld the newly empowered girl tear threw the demon horde as one tares paper but more were coming and the villages were accustomed to protecting their village for the demons and they joined her and fought the demons off but not before one the demon killed the girl.
The elders noticed this and with the magic's of the earth blazing though their veins the saw the essence pass for the girl into another girl of the village even as the last of the demon was destroyed. So, it was they found that they had quite by accident created a line of warriors who protect the earth for the many thousands of years to come.