A hallucinogenic plant custom-designed in the world of The Sundered.
SPOILERS. From this point on, you are entering spoiler territory.
Motherwater lacks any kind of plant life that humans could use to reach an altered consciousness. Naturally, people wanted something for that purpose, and so required their claimed Sundered Ones to create an effective plant for them – something they could use recreationally without ill effect.
Humans wanted the dream leaves as a form of recreation and escape; the Sundered Ones latched onto them as a means of influence. While under the effect of the smoke from burning dream leaves, a human’s body goes through extreme stress (adrenal glands, electrical activity, heart rate, etc.), allowing them to be briefly enhanced on a neurological level. In that state, they can almost hear Sundered Ones’ thoughts. Not quite, but close. It allows the Sundered Ones to influence dreams.
Of course, as Aakesh tries to tell Harry , anything that relieves stress in a human can become addictive; it isn’t a simple matter of physiology, but of how the human soul works. Addiction is a much bigger problem than mere chemistry could ever address.