The final battle in the Manor is a mind game

 "The terror of Zankou has forced the sisters into an unprecedented situation. They cannot destroy him with a simple spell. They cannot defeat him with strength. Zankou is so powerful that, to defeat him, they must sacrifice something greater than their lives: themselves.

The final battle in the Manor is a mind game. They use astral projection to deceive Zankou, while their real bodies chant the ultimate spell to destroy The Nexus – the Energy Intersection just below the house. They choose to destroy their own power source to destroy him.

And the final moment, when Agent Keyes announces their “death” and the door to the Manor is closed by their “ghosts,” is not a glorious victory. It is a haunting farewell. They have saved the world, but they have given up their identities, their histories, their lives. It was the price they had to pay to escape an enemy they couldn't defeat in the usual way.

In short, ""Something Wicca This Way Goes...?"" was a perfect ending to an arc in Charmed, and Zankou was the perfect catalyst for it. He was terrifying not just because of his power, but because he succeeded in forcing the world's most powerful witches to admit that sometimes, the only way to win is to disappear completely."