A hallucinogenic plant custom-designed in the world of The Sundered.
SPOILERS. From this point on, you are entering spoiler territory.
A character mentioned in Love Makes Whole, a coll... 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 Powerful but broken creatures, enslaved to keep mankind aliv... to create an effective plant for them – something they could use recreationally without ill effect.
Humans wanted The Dream include any people who live between dimensions and... 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 is a Sundered One, and the first-created offspring of... tries to tell Harry Iskinder is the main character in the book, The Sunder... , 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.