Time travel is utterly fatal. If you travel back in time, even by a second, you will die. No special effects. No horrifying bodily trauma. No marks, no mess. Just dead.
Now, this was wonderful for the weapons industry, but not so great for the "going back in time and seeing the past" crowd. The best that you can do is set up some probes and then call them back, but time travel messes up electronics something bad. You won't always get clean recordings, and you won't even always get your recorder back. You can always let your recorder take the slow way back to your time in order to avoid giving it a nice wash in the timestream twice, but the longer it'll take to reach you the more time it'll have to be destroyed in some other, more mundane, manner.
Thankfully, there's a wonderful solution. Vampires are totally dead, contrary to every scientific law which would like to have a word with them about animated corpses that sustain themselves on blood. And while nobody was expecting them to actually exist, it wasn't two minutes before the secret was out before some crazy scientist said what all of the other crazy scientists were thinking. "Let's send them back in time."
It wasn't as simple as grabbing vampires off of the street and throwing them into the past. There had never been very many, and there were even fewer now. An even bigger problem was that you couldn't trust them. As soon as a few had been collected, though, a solution to both problems was easy to fashion. The crazy scientists recruited crazier personnel, trained them for the missions that they would shortly undertake, and turned them into vampires.
Of course, how did vampires come to exist in the first place? They should be impossible. And to lesser beings, they are. Or were. And would be. But time travel makes temporal tense a tricky thing. Billions of years from now, the universe will be coming to a close. Our descendants will have scratched the ceiling of possibility and uncovered every secret. They were content, because it would have been useless to be otherwise. But they were not going to leave the doable undone. And though it was still impossible to travel through time and live to tell the tale, it was slightly less impossible to create something that could act, think, persist, and yet be dead.
It's funny what the universe will let you get away with, in the end. Our descendants created vampires, and they sent the first vampires back to the dawn of history. Not to do anything that would be considered spectacular in any other circumstances. Just exist, and keep existing, and bring more people in as necessary to keep it going. But history was rewritten by that simple, continuous act. Not that our one-time descendants minded. They had done all that they could. Their history was full. But by granting this gift to their dim and distant ancestors, they could create a history fresher and more full than their own, a history with billions of years of time travel against a universe of light, not huddled around the last remaining, manufactured stars, waiting for the darkness to come in.