That staff the Elder Brain gave you can be used to extract your brain with the clawed end, and then dissolve your body into a sustaining brine fluid with the poison in the spike on the other. Drop your brain into that pool with a couple other Illithid brains and you'll become a new Elder Brain in no time. A new kind of eternal life for you and those worthy to join you in that perfect unity.
"Yeah I think I'll stick with the whole growing old and dying surrounded by my loved ones plan if you don't mind." Harry said sarcastically, he got the sense the voice within him had mentally shrugged as if to say to each their own. Although depending on whose brain he got to join with an eternal brotherhood with a few chosen loved ones didn't sound that bad. Shame he would have to transform into a giant, tentacled, mind control brain to enjoy it though. After a short time he continued as he stood up.
"We should get back home." Harry said, "I don't know how long we've been gone but it still feels like summer so it shouldn't be that bad. Once we get to Hogwarts we can start looking into ways to either hold off or control this whole ceremorphosis process. And then we can start preparing to take down that Elder Brain and keep our world safe from Illithids."
It took a day to take us to the colony, another two for the tadpole to properly fuse with our mind, then only a few hours to get back here. Said the voice. So we haven't been gone that long.
"Three days huh?" Harry commented. "So we'll have to put up with another full day of dealing with Aunt Marge. That sucks. Hey what do you want to be called anyway? I can't just keep thinking of you as my new inner voice."
It doesn't really matter, you'll probably be the only one to ever speak to me.
"You called us something like The Eternal Survivor right? How about ESHarry then?"
Esh-arry. I kind of like it. Yeah that works.
Authors post-script: yeah I know I'm playing a bit fast and loose with Mind Flayer lore here but bear with me here, its all in the name of progress.
Review and critiques are most welcome though I ask you all keep them to the subject of the story and not launch into some 10000 word rant about DND lore or how terrible JK Rowling is or what have you.As Harry made his way out of the woods back to number 4 privet drive he considered the long, spine like, metal staff the Elder Brain had given him. The thing seemed to naturally resonate with his magic which was good. It was also meant, according to Esharry, to pry out his own brain so he could transform into a giant monster which wasn't so good. Worst of all it wasn't the sort of thing that Harry could imagine any muggle walking around with so to his aunt and uncle it would seem to be something magical and therefore forbidden under Uncle Vernon's house. Harry was tempted to just throw it away, but he suspected that he would be expected to one day go back to the Illithid colony to give an account of what he had been up to and showing up without the staff would be very suspicious. Eventually Harry had an idea.
Just before he left the woods he lifted the staff up before his eyes and concentrated on it. Then he made a quick motion with it through the air and said, "Diminuendo." In a commanding voice. Immediately the staff began to shrink in Harry's hand for as long as he focused on it. Once it was no more than about five inches across, Harry released the spell. Harry was quite pleased that it had worked, he had thought the staff could function like a wand and was glad to have confirmed it, this was also the first time he had ever cast a spell on a wand he was using. He slipped the shrunken staff into his pocket and slipped back into the Little Whinging neighborhood.
Harry had only crossed a few streets when he was greeted by a very odd sight. There was a man standing on the corner of one of the roads looking about anxiously and he was quite obviously a wizard. While he was wearing muggle clothing it was obvious that he didn't understand them, his jacket looked about forty years old, his pants and shirt badly clashed in colors and he was wearing his suspenders over his coat rather than just over his shirt. When the man saw Harry his eyes lit up. He lifted his wand into the air and shot a bright burst of sparks off like a firework while he shouted: "I found him! I found him!" In a loud excited voice.
Harry might've been worried as the man ran over to him but he never pointed his wand at Harry or made any move which seemed threatening. In fact the man looked profoundly relieved to have seen Harry. No sooner had he arrived than there was a loud cracking sound and several other people suddenly appeared around Harry. Most of them quickly fanned out to surround Harry protectively while looking around him as if expecting an attack, while two of them closed in on Harry himself. Surprisingly Harry recognized them both, one was Cornelius Fudge, the Minister of Magic, in his distinct green bowler hat. The other was Professor Albus Dumbledore, the headmaster of Harry's school, wearing a bright purple and silver set of robes and sporting a very long white beard.
Harry was quite surprised to see the most influential man in magical Britain and perhaps the most powerful wizard alive in the world bearing down on him with looks of concern and relief on their faces in equal measures. Minister Fudge stopped short of Harry and began wiping sweat from his forehead while Professor Dumbledore began flicking his wand over Harry in the same way he had seen the school nurse, Madame Pomfrey, do to him a few times and so was likely checking to see that Harry was alright. For a moment Harry panicked thinking that Professor Dumbledore must know that Harry had been to another world and become infected with an Illithid parasite, but the man was relieved by what his magical scans told him so that didn't seem to be the case.