|"Is this all real? Or has this been happening inside my head?"
The topic of this article is of a real-life subject that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really exists), and, as such, details contained in this article may differ from real world facts.
Moonseed is a flowering plant in the family Menispermaceae, native to eastern North America, which seeds resemble a crescent moon. All parts of these plants are known to be poisonous. It is used as a main ingredient of the Moonseed Poison.