Source: Bestiary 3, pg(s). 38
Brainchildren are rumors brought to life, springing out of thoughts and fears about hypothetical killers.
Brainchildren have no inherent appearance—the creatures always look like their prey expects them to look down to the items they appear to possess. When conflicting beliefs exist, the brainchild itself chooses the form it will take in that moment.
Born of rumors, the abilities brainchildren possess also come from rumor. If local gossip ascribes to the brainchild an immunity to a particular kind of magic, that brainchild will have that immunity. If legends tell of a weakness, the brainchild will possess that weakness. Vitally, the foes faced must believe in those strengths and weaknesses else the brainchild will lose that facet of itself for that fight. A defeated brainchild will reform if any still truly believe the brainchild exists.
Brainchildren can appear in any region where superstitions and fears about remorseless killers grip the populace strongly enough. The rumors will often begin with a death from a phantasmal killer spell that is ascribed instead to some superpowered killer. Brainchildren cleave strongly to this tale of murder and are often only driven by a desire to terrify and kill.