I own neither Buffy verse nor Doctor Who. If I am mistaken in this I will happily accept either or both of them.
We are all told of how the shadowmen took the girl and put the demon into her. But the truth is far more complicated than that. You see the shadowmen didn't have a clue as to what they really did. Sure it was a demon, if you know demons were aliens. What might surprise you is that when the created the first slayer it was not the first time they had infused the power of a demon it is mealy the first time it was ever passed on. Much to the surprise and trepidation of the shadow men, because they really did not know what they had done.
So what did they do. Well, if you must know the slayer was created during a very special time a time that ever was or is or will be. So when the watchers do their speech it is true but so is are the scientists. If fact if you were to go back in time the scientist would be right. But that does not mean the watchers are wrong it just means they know of an Event that devastated the universe. This Event, well it was the Time War. It raged throughout the cosmos most the demons come from that era. Spewn out from holes of the larger battles monsters of horror that could only have come into being is such a horrible time. The hellmouth an entrance into this time, despite the Oncoming Storm sealing of it.
But that’s not the point the point is that when the shadowman took a being who already had the title of Slayer and forced her to augment the only vessel that would accept her they inadvertently created the slayer line. It was never their intention to create a line. Hell, it was never their intention to use any girl let alone a young teenage one. But the spirit would only accept the young girl and so they allowed Sineya to be the recipient of the spirit.
This spirit was of a race that had the most miraculous ability when they came close to death they can cheat death and form a new body in a process they call regeneration. It is this ability that allowed the slayer line to be created. It is this ability that attracted the first to the slayers, when it was freed from it black hole prison created so ever long ago. Before even the great people of Gallifrey took on the mantle of Time Lords back when the universe was so new, no race currently living could survive back then. Back when it, the being now called the first evil, had reigned supreme with the power of the key. It wanted that back. But they key is not something one can just take. They key is bound to only one master at a time. But it was not going to be deterred even after its humiliating defeat at the hands of said master. It had time all the time in the universe and when the slayer was down when she was faltering it would strike and it would gain the key and the ability to form a body however and whenever it desired. Then then it would be the master of all the universe, and no one could ever defeat it again.
This is a short introduction to a idea I have. I do not know if or when more may be forthcoming. If I do write more it will be separate story. Thank you for reading.