Idea: Displaced Time Travelers Taking the Slow Way Back

Yes, more time travelers. Yes, more vampires.

But no stable time loops this time, oh no sir.

Where did they come from? Or when? A long, long time hence, of course. Or tomorrow, or somewhere in-between. But on an otherwise-routine trip their machine malfunctioned and they found themselves stranded in the past. A few seconds was all that it took to confirm that help wasn't on its way. If it were, then it would have arrived before they even discovered the malfunction. That meant that they were totally on their own.

They refused to accept this.

The machine was truly a masterpiece of technology, far beyond anything that we could conceive. That it was unable to return them to the past was a horrendous spat of bad fortune, but that didn't mean that the whole machine was down anymore than a broken engine in a car means that the brakes and battery are also irreparably damaged. The machine was built to affect the biological structure of things in order to produce food and adapt its passengers' bodies to whatever circumstances they found themselves in.

Its capabilities were far beyond what was considered to be required for most missions, but the quasi-singularitarian technology of their day was even more advanced, and it was no trouble at all to outfit the machine with such potential. It was the passengers' one spot of good luck (though perhaps it was no luck at all, but had secretly been intended for just such a contingency as this, some of them wondered) and they used it to re-engineer themselves biologically. Their memories were doubled in their junk DNA, their lifespans were lengthened, and they were given the ability to infect others with this same condition so that their own DNA would be altered in select places. Each generation would carry the memories of the next.

Unfortunately the process didn't work completely beneficially. Errors crept in, consequences of dabbling in sciences a little too advanced than they were familiar with. Their diet, for example, was totally screwed up and they find it impossible to digest anything more substantial than a fine paste. Sunlight will give them a massive case of sunburn within seconds, and prolonged contact with cancer them up like you wouldn't believe.

Their aim is to survive the long thousands of years between where they were and where they had come from, and they're prepared to do it by any means possible. They've been playing with history, pulling these strings and cutting those ones, all to accelerate our advancement (and doing about as good of a job as anyone who didn't get a PhD in historical puppeteering). They don't care who they have to kill, how many lives they have to destroy.

Because in the end? It won't matter one bit.

History can still be rewritten. They're in the alpha timeline, if you want to call it that, the one that simply had to come before there could be a timeline where they were rescued. And so long as they're willing to pull the trigger, they can be the cavalry they knew that there was no point in waiting for.

(note: to be clear, the "vampire" concept is as novel to them as it is stereotypical to us. they messed up history like you wouldn't believe and there were no such stories in their timeline.)