There are a few antonyms for the word "aforeknown," which means known beforehand or predestined. Some possible antonyms include:
1. Unexpected: this word implies that something was not foreseen or predicted beforehand.
2. Chance: this refers to something happening by luck or opportunity, rather than due to predestination or foreknowledge.
3. Unforeseeable: similar to unexpected, this refers to something that was not able to be predicted or foreseen ahead of time.
4. Improvised: this suggests that something was done on the spur of the moment or created without a plan, rather than being predestined or already known.
Overall, antonyms for "aforeknown" are words that suggest randomness, surprise, or the absence of a predetermined plan or fate.