I'll interject briefly here to explain Nico's plan in ordinary language, since it doesn't look like Nico is ever going to.
Humans have three souls, which alchemists think of as the Animal soul, the Knowledgeable soul, and the Divine soul (except they usually use Latin or Hebrew or something, because alchemists). The Animal soul is mostly just about instincts; it doesn't persist after death, and alchemists don't usually pay that much attention to it (though admittedly Nico did do something pretty clever with it when he took over Kyle's body). The Divine soul is the part that gets reincarnated; when somebody says they were Cleopatra in a past life, what they mean is that they have same Divine soul she did. This is not as useful as you'd hope. You'll tend to have the same sorts of values and intuitions Cleopatra had, and you might get a few subconscious influences about things that were emotionally important to her, but even if you get in touch with your past lives you won't suddenly learn how to speak Demotic Egyptian.
Except that there's also the Knowledgeable soul, and it's a puzzle. Because, you see, if you do exactly the right things in exactly the right way at exactly the right time, you actually can learn to speak Demotic Egyptian from your past life. It's hard to do, and doesn't last very long, but you can, and lots of people have (at least by alchemy standards). So all that knowledge is clearly being stored somewhere: it doesn't dissipate the way the Animal soul does, and doesn't get passed from life to life the way the Divine soul does; some secret third thing happens instead.
(There's also a whole separate line of evidence involving ghosts, but it's complicated and not relevant to Nico's plan so we're mostly going to skip over it. tldr most ghosts sightings are fake but ghosts are real, and you don't necessarily have to die to make one).
Not everything in those last few paragraphs is true, but it's pretty close and Nico believes it so let's go with it for now.
Meanwhile, a sad fact about alchemical knowledge is that it gets lost a lot. One person figures something out, but then before they figure out how to tell people they blow themselves up trying an ill-considered reaction, or breathe too many mercury fumes and poison themselves, or just melt their brains trying to eff the ineffable. Or, to be fair, they reach a certain kind of enlightenment and conclude that everything is perfect as it is and there's no need to teach anyone anything -- but those people might as well be dead, from Nico's point of view. Not everything disappears that way; a lot of what we now call "chemistry" is alchemy that went mainstream. But the weird, hard-to-explain stuff gets reinvented over and over, mostly by people who are one insight away from catastrophe. Hardly anybody believes in the occult anymore, and this is a lot of why.
But all that knowledge isn't really going away. It's still Out There, and Nico figures he knows where. His plan, if you really boil it down and gloss the occult parts, is to open the gate that separates all those lost souls from the material world, reach in, and gorge himself until he knows every alchemical secret anyone ever learned. If a normal alchemist tried this they'd probably just ruin their Divine souls and then die, but Nico's been preparing himself for years. He might not be able to learn everything he wants to learn, but he'll live to try again. The stack of souls that calls itself Kyle Merrill is doomed, but Nico will be OK.
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