Dream manipulation as in manipulating the dreams of other people to push nefarious agendas
If you look around far enough you'll find people with tales of having their dreams manipulated by malicious magic practitioners, and certainly there's folk charms created to do this kind of thing; but as with most things in magic, there's no solid evidence to back up whether this really works. But if it works, there are also anecdotal reports that suggest that you can just use warding magic to keep your dreams from getting meddled with, so there's that.
That said, I've also seen a number of people having nightmares that most probably relate to personal trauma and stress assuming that some malicious actor must be trying to attack them with bad dreams. And of course, ignoring your stress and trauma in favor of thinking you're being maliciously targeted in ways you most probably aren't is super unhealthy.
Now, you said "to push nefarious agendas" so I'm thinking you probably mean this in a conspiratorial context. To that, I'm going to say that there is no reason to think that there's some secret conspiracy doing anything of the sort. In fact, it's kind of the opposite, because the people who claim there's an evil conspiracy giving people nasty dreams are inevitably also pushing it in the context of your typical Protocols/blood libel/witch panic redux conspiracy theory.
This isn't to say that no one is having dreams that seem to correlate with actual terrible things going on in the world. But I don't think that this is because of dream manipulation, but rather because people just have these kinds of dreams sometimes. There's no reason to tie it into some grand conspiracy narrative, especially given what we know about the origins and intentions of these grand conspiracy narratives.
