The phrase "descends upon" is often used to describe a sudden arrival or occurrence. Antonyms for this phrase might include "leaves," as in a departure or exit, or "recedes," to describe a gradual retreat or reduction. Another antonym could be "averts," indicating a deliberate effort to prevent something from happening. Other possible antonyms might include "ascends," suggesting a upward movement or growth, or "evaporates," describing a dissipation or disappearance. Ultimately, the choice of antonym depends on the context and intended meaning of the phrase "descends upon.