Parasitism (寄生 Kisei)[1][2][3] is the act of a Crimson Denizen going inside a Torch or Mystes and taking control of it, using it as their own body.
"Village of Jestful Slumber" Mare was able to take control of Mystes through her Unrestricted Spell Gemeinde. She had last parasitized the Mystes of the Treasure Tool Papagena. "Spiral Organ" Leanan-sidhe also operated as "Corpse Retriever" Lamies through the Torch of an old man.
Background
Though Mare was a small and weak existence, she had the ability to sneak inside Mystes. The parasitized Mystes does not notice the Crimson Denizen inside them. Due to her small existence and this ability, Mare had been able to escape detection by Flame Haze.[1][2] This Parasitism was possible because Mare possessed only a small amount of Power of Existence. She had been parasitizing Papagena's Mystes until she arrived at Misaki Municipal High School, where she switched to Yūji Sakai.[2] Mare's method of taking over Mystes was to eat away their minds using dreams from Gemeinde, like she had done to Papagena's Mystes.[4]