The phrase "in event" connotes something that may happen or is scheduled to happen. Its antonyms are words that indicate something is certain or unlikely to occur. The antonym for "in event" can be "immutable," which means cannot be changed or altered. It can also be "inevitable," which means bound to happen, or "definite," which means certain to happen. Closely related antonyms are "unlikely," meaning not expected to happen, and "impossible," which means cannot happen under any circumstances. Thus, the opposite of the phrase "in event" is a variety of words that suggest something that is sure to take place or unlikely to occur.