This Sphere contains 2 different concepts: decay and probability. Decay meaning death, rot, wear and tear, erosion, forgetting, etc. Probability means the likelihood of something occurring or not. At low levels it allows a mage to identify weak points and make very good guesses. At higher levels it allows curses, breaking or preserving objects, changing the likelihood of something, injuring people and erasing ideas.
In the first 2 editions of Mage, the Syndicate took a strong interest in this Sphere (probability). In Revised Ed. and Mage 20 the Syndicate favored Prime. The Euthanatos are attracted to it too (decay).