Help with brain themed dungeon

[Hex-Toliver]

As the title states, I'm trying to come up with a dungeon based around the brain. Just to give some information about what i'm sort of picturing and what the setups like. Currently i'm playing in a game where the 5 of us take turns dming, we're playing a pirate sort of game where we sail about on a boat from one island to the next, with all the misadventure in between. We basically take turns rolling up a 1-2 session dungeon/encounters that we dm, and it's my turn next week. For some reason i got struck with the idea that I wanted a dungeon based around being inside a huge floating head, its being an uber powerful magical or psionic creature. I wanted possible rooms to be projected memories or some sort of delving through this things memories as the layout, or some sort of pocket dimension effect in the dungeon. I'm thinking the end goal of said dungeon is some sickness or brain parasite that is infecting it. I'm making this for a very cheesed and very optimized 5-6 man 5th level party, not trying to purposefully TPK but the last DM's dungeon was way too easy for the party, and i want a true challenge. I know this is all over the place but if anyone has any thoughts or suggestions on the matter I'd truly appreciate the effort.

[RedHydra]

Having a hard time picturing it but I know that Intellect Devours if played right are amazing creatures. One of my GMs did an adventure with multiple separate rooms connected by fog doors where you needed to roll a DC 20 Will save or be sent to a random room within the dungeon that could each have their own puzzle / brain themed area. If you want a potential lasting effect on the party you could have the creature apply this template to one of your party members.
https://www.d20pfsrd.com/bestiary/monster-listings/templates/psychoplasmic- creature-cr-1/
Other than that the best way to make adventures is to make a bunch of rooms and fill them with monsters. Aberrations regardless of CR will pose a challenge to your party plus by the description you've given of the creature I would wager it would probably be an aberration too.
Hopefully you got some sort of an idea from my jumbled up mess

[Hex-Toliver]

No, that did help. I'm thinking maybe that template could be some sort of temporary reward for solving one of these brain rooms. Think a powerup before the boss. And the random room teleporting will be a neat trick too, to split that comfortable party up. Of course their will be nasty mind bending aberrations, i was thinking of using the Seugathi https://www.d20pfsrd.com/bestiary/monster-listings/aberrations/seugathi as the end boss, possibly with some maddened minions or something. But i like adding RP challenges and puzzles to my dungeons, so i want that template to be earned by doing something more than killing.

[Dave Justus]

What if you made it a memory theme. Replay some of the most interesting significant encounters the party has had in the past, but make them different (obviously the challenge rating adjusted, but add in weird templates and or abilities as well). The parasite is building the encounters from the PCs memories. You could also add in a Ghostbusters 'choose the form of your destroyer' for the ultimate battle, although making that work in an RPG might be a little difficult.