The tadpole burrows into its victim’s brain, quickly consuming much of the gray matter and replacing the consumed brain with its own squalid tissue. In effect, the tadpole melds with the uneaten lower brain stem of the victim, killing all remnants of the personality and spirit of the victim, while leaving the physical body alive for the tadpole to use as its own body.
I never touched any of the later WotC-editions, but seeing from an AD&D (2e) standpoint, the whole Absolute crisis revolves around extremely modified tadpoles, specifically bred at Moonrise Towers, that conserve much more of the host's personality and memories than your standard ceremorphosis ever would. Fantasy aspects aside,
Naegleria fowleri can eat your brain. So, they will kill you in pretty much the same way an illithid tadpole would, annihilating any vestiges of your mind. That's why everybody says illithids are eating souls and why you can't just reverse ceremorphosis with a simple spell. The resulting standard mind flayer, however, also needs about 10-20 years to learn how to properly illithid according to the original lore. They don't retain any memories, nor do levels, abilities, proficiencies, powers or traits, like elven longevity, transfer from the host to the resulting mind flayer. They spend the first two decades of their lives in momma's brine pool, learning how to swim before they can levitate. It's however not unusual for illithids to go rogue after their motherbrain dies and enjoy their freedom, like Omeluum or the Emperor. We only gradually find out during the game that there's something strange going on with our tadpoles, which is why I thought the change of theme and tempo is fine. We won't die within the usual few days, but when we're supposed to turn.
They way I took it, the Emperor may have been a standard mind flayer after his transformation, but he also was quite a powerful adventurer before that. Omeluum developed its own non-evil personality, regardless of who it was before, but the Emperor may have held some powerful artefact that preserved at least some of his original personality which made him special. Or yet again, the illithid he became simply got a bit weird with age. In the original game, mind flayers live about 125 years.
Balduran vanished sometime during the 11th century of Dale reckoning, which would make the Emperor a really ancient specimen of his species. Him previously being an elf should not affect his life expectancy of little over a century.
To me he came across like this illithid-supremacist control freak whose plans were hit-and-miss most of the time and who had delusions of his own and his species' grandeur. In AD&D mind flayers are of "genius" intelligence. That's 17-18, still easily within range of a human. Neurology may play a part, of course, and the Absolute isn't your standard elder brain either.
I also couldn't figure out during my playthrough if Stelmane was turned by order of the Absolute or due to the Emperor's own experiments. Maybe he thought turning her could cure her, or did she suffer her stroke because the first half-flayer transformation failed? The question remains if that happened on Gortash's order to cripple the Knights of the Shield (but keep Stelmane as a figurehead), or by the Emperor's own botched design.
Taking the illithids-only restriction at face value, my Tav would have preferred, after getting the Foundry Gondians to fix Karlach's heart for good, to give the Netherstones to Omeluum, free Orpheus and kill the Emperor on the spot for even suggesting treason.
I didn't try to use anything else on Orpheus' chains, but when Lae'zel struck with the hammer she shouted "Vlaakith's will be done!" I blame the Zaith'isk.
