The Watcher Council is very stingy with access to their records, and the vast majority of the Diaries are sequestered for internal research only. That makes any discussion of their history, the war against the demons, and individual slayers challenging to say the least. We know (from studies of the Summers Papers) that the First Slayer arose in prehistory; some sort of attempt to create an immortal warrior capable of meeting the demons on their own terms. Beyond that, speculation runs rampant.

We can reasonably assume that, because slayers almost always operated alone and out of sight, any one of them badly wounded but not slain outright would likely not be found in time to save. Even in eras with adequate medical care, it's unlikely that assistance would arrive in time. Unfortunately, this is the best most scholars can do with the available evidence. Investigation into individual Slayers may yield greater details, but until the Council opens more of their records, most historians are stabbing in the dark.

No pun intended.

There's a lot of interesting debate on this topic. It was widely believed for a few years that Kendra was the first "second" Slayer and that Buffy was the first Slayer to die and be revived. Otherwise, there naturally would have been multiple Slayers by the time of Buffy's first death. This belief was called into question when Buffy died a second time, and a new Slayer was not called. Ultimately, the answer lies in the records kept by the Watchers, and many of those were destroyed when their headquarters was bombed by the First Evil, so we may never know.

Is no one going to mention that this clearly breakes the subbreddit rules?

This is clearly less than 20 years old.

It's true that Buffy was probably origionally called more than 20 years ago (the exact timing is either 1996 or 1992 depending on who's accounts you believe) but there is no question on the timing of her resurrection. It is within that time frame as refrenced in part one of David Grossman's two part journal, Bargaining.

Your use of the word "first" is problematic given the apparent fluidity with which the Slayers navigate time. Buffy Summers, for example, was able to communicate with the First Slayer at least twice; and after the destruction of Sunnydale found herself in the 23rd century fighting alongside Fray. It needs to be stressed that the multidimensionality of Buffy's universe involves not only space but time. Angel spends centuries in a hell dimension but ages not at all. In her adventures Buffy meets numerous beings who are apparently immortal, like the woman who watches the Watchers and gives her the scythe; Glory the god; and of course, the vampires. Do first and last have any meaning in this context? When the last Slayer can use magic to communicate with the first, it creates an infinite loop which nullifies the riverrun of time. History itself loses all significance.

Indeed, I would suggest that /r/AskHistorians is the wrong subreddit to look for an answer to your question.

I believe Ms. Summers is the first.

Restarting the heart has only been a practical discovery since approx. 1954, and CPR has only been publicly taught since about 1972; well within the lifetime of Giles and most of the Council. If a "second slayer" scenario had occurred before, surely it would have been covered in Giles' training.

/u/melkahb makes the very good point that most Slayers operated with virtually no support structure; I would add that any enemy who killed a Slayer was most likely still in good shape to deal with any conventional mortals afterwards, so recovery of a "mostly dead" Slayer in time for CPR would have been extremely unlikely.

As slayers were meant to fight alone, it is relatively common knowledge that they have not. Buffy, being as defiant as she was, insisted on endangering her Scooby gang, thus putting them at risk. First, Xander saved her, per The Master in Sheep's Clothing vol 2 and then Willow saved her in Tara's Anthology. It seems to me that these were her two must trusted scoobies. Since previous slayers didn't have a Scooby "gang" as it were, we can generally assume nobody was there to revive them, as it is clear the watchers council prefer her death to the confusion that happened with Kendra and then again with Faith. Best that they all be dead rather than some weird evil power struggle.

After finding out the possibility of having two concurrent slayers, why didn't the Council then make a practice of killing and reviving slayers in order to produce more? I realize this would have been unnecessary after 2003 when all the Potentials were activated, but it's hard to believe this knowledge wasn't used.

Probably not, as the Slayer line has existed for far too long. Someone could easily have "died" and been resuscitated. However, this has not happened within the span of the modern Watcher's Council records and unfortunately they are nearly the only source of reliable records on the slayer.

However, it seems that the mechanism of the spell that calls a new slayer "follows" the slayer line as opposed to reacting to the death of the Slayer. What this means is, while there may be two slayers walking around, only one of them can trigger the call of a new slayer by dying. We know this only because Buffy Summers has actually died twice. However, only her first death triggered the call, resulting the slayer Kendra. Kendra's death continues the line by bringing us the Dark Slayer. Meanwhile, Buffy Summers' life or death continues to have no bearing on the call.

So if it happened before, they eventually died and the line continued on as a one at a time affair. It was important for it to happen to Buffy, as it almost certainly provided "proof of concept" for the eventual alteration to the spell, allowing anyone who could access the power of the slayer to do so.

Which brings us to what the Slayer is. The origin story is so old at this point that we don't have any of the hard facts. But it is generally agreed that the Slayer was likely the creation of Sumerian tribe with particularly strong access to magic, 8-10 thousand years ago. Every Helmouth on earth would have been open at this time, and dimensional barriers relatively weaker all around. Demons preyed on humans regularly, and the bigguns threatened cities. The Slayer was a particularly successful attempt at a countermeasure, that proved so effective that it has been maintained in some form for the better part of human history.

The nature of the spell itself is still very much in dispute and heavily discussed. But it's generally agreed that it involved the binding of a demon essence or soul to a young girl, that sought to replicate itself when the slayer died. The demon in question has yet to be identified, but is suspected to be a full blooded predatory demon (as in a demon that fed on demons) due to the fairly consistent predisposition Slayers have had for demons as well with unnaturally heightened senses and capacity with regards to finding them.

It is actually argued that Jesus was the first slayer. If this is the case, then that would make Buffy the second revived. As for how it's possible, this is not /r/askscience. While the question is relevant, and related enough, I think you'd get a better answer there.

source