The energy from the universal forces of Dynamism, Entropy & Stasis. First defined on pg. 131 of Book of Secrets.
For my games it could be the Resonance of a place or object that does not come from mages’ actions or the actions of other Night Folk but from umbrood or forces in the universe. Synergy can affect mages, especially if it aligns with or opposes the mage’s Resonance. It could also be Resonance of any sort outside the mage that affects a mage if it is the same or opposite of the mage’s own Resonance.