In a system where there are multiple planes of existence (the center one being ours), I want my human (mostly - not important for this though) main characters to interact with forces from the other planes. However, the creatures in those other planes and their ways of life are not perceivable by humans or these characters. One way my main group could encounter trouble would be in a classic man-vs-man dynamic - where adults working for those in the other planes do things to harm them.
So the creatures in the other planes need to communicate with humans in our plane. How would this occur? Dreams have come to mind, but then I ask how that would work. "Dreams" just being the reason seems too simple. Mind-control is also a broad reason to tackle. And there is telepathy, but then I ask how could that be elaborated on. What interests you, and how could this make sense?