Complex sentence: The illithids, who know much that is hidden, tell a somewhat different story, claiming that Mellifleur interrupted not just one such apotheosis but many, usurping the ascension of many gods on many different worlds.
Split into simpler sentences: The illithids, who seem to know much that is hidden from others, tell a somewhat different story. They claim that Mellifleur interrupted not just one such apotheosis, but many, thus usurping the ascension of many gods on many different worlds.