Zankou

 "Zankou was one of the most intelligent enemies the Halliwell sisters had ever faced. Rather than resort to brute force, he chose a more ruthless strategy: he attacked their greatest weaknesses - compassion and guilt.

Zankou began to ""treat them like people"" by exploiting their painful pasts. He summoned the spirits of innocents the sisters had failed to save in the past, along with those he had killed himself.

These ghosts did not attack, but remained silent, turning their home into a hell of torment. This vicious psychological attack paralyzed the sisters, leaving them drowning in guilt and self-doubt. By weaponizing the past, Zankou proved himself a formidable opponent, nearly destroying the Three Magicians from within."