There are tales of a creature that appears to people in their dreams. One that appears to be a simple marionette on strings that never seem to end, with glassy yet lively eyes and a carapace made of sanded wood. It claims to hold the key to happiness for all it meets, and is a softspoken and noncombative entity, all the more to enact a quiet form of cruelty. Every moment spent consciously listening to its voice extends the time you would spend in the "rapid eye movement" phase, ensuring it has a foot in the door for as long as its allowed your ear, and leaving people who listen for too long lethargic and restless should they wake... If it lets them wake.
The creature, known as Mr'yon, spends time in this connection between its mind and a chosen victim breaking down the victim's resolve, manipulating and weaving different dreamscapes that seem real at a glance, often favorable enough to overlook the odd detail to lull them into compliance or helplessness or aggressively stagnant to break any hope of escape. All of this is in the service of siphoning their soul's energies for itself.
If allowed to go on long enough, it can manipulate them even while awake, hiding in their subconscious and piecing together stray thoughts into new ones to suit its goals and eventually spreading its influence to those the victim knows and eventually, after having gotten everything it needs and they could possibly give, killing them the next time they fall asleep.
Every mastermind however, has a fatal flaw, and Mr'yon has two. The first is that its powers by necessity tend to induce lucid dreaming, with it taking great pains to obfuscate this. If this is discovered, it becomes possible for a strong willed individual to shut the demon out, but this is a temporary, case by case solution.
The second one, used to seal it away, is that due to the demon's powers blurring the lines between the dream world and reality, it can be forced into an appropriately shaped vessel if its victim falls asleep with one on hand and grabs the demon as they're forced awake. Once sealed in this way, its curse on living victims will be broken, and it will become little more than a moving toy that can be safely contained.
However, if left to its own devices, it might just find a way to escape, where it can modify itself into a more ideal shape to feed on more victims and, chiefly, goad its hunters into destroying its body so it may return to the dream world.