There's eternal opposition between yin and yang.
No third party at all, but treason occurs sometimes.

Accept your dark side, understanding it will help you to move with the light. Knowing both sides of our souls, helps us all to move forward in life and to understand that, perfection doesn't exist.

Said the man to the sun, "How I wish you could shine your light on every day of my life!"
Said the sun to the man, "But only with the rain and the night could you recognize my light."
---Domaccan poem, translated by Chevalle

We've heard history for centuries. It's time for herstory.

On the blank leaf glued to the inner back cover I drew the double curve within the circle, and blacked the yin half of the symbol, then pushed it back to my companion. 'Do you know that sign?'

He looked at it a long time with a strange look, but he said, 'No.'

'It's found on Earth, and on Hain-Davenant, and on Chiffewar. It is yin and yang. Light is the left hand of darkness... how did it go? Light, dark. Fear, courage. Cold, warmth. Female, male. It is yourself, Therem. Both and one. A shadow on snow.'

The dance between darkness and light will always remain— the stars and the moon will always need the darkness to be seen, the darkness will just not be worth having without the moon and the stars.