Ozma's exact nature is unknown, but because it is found within an "eidolon cave", it can be speculated to be an ancient eidolon whose power the summoners couldn't harness, and it was thus locked away. Over the many years since, the legends that gave birth to Ozma would have vanished, and the people who used to tell them would be extinct, with the result that Ozma loses whatever form it may once have had and only appears to the party as a swirling mass of energy.