Eonian, meaning lasting forever or eternal, doesn't have many commonly used antonyms. However, a few words that could be considered as antonyms are temporal, transient, and fleeting. Temporal refers to something related to time and limited by it, opposite to eonian, which means eternal. Transient, meaning not lasting long, and fleeting, which refers to something that passes quickly, are both completely different from eonian's long-lasting nature. Other words that could be regarded as antonyms for eonian are mortal, ephemeral, and perishable, all which describe things of the earthly, limited world that have no lasting value in comparison to things that are eonian.