viscid sage
Everything about the game hints and even has unlockables hidden behind killing the brain boss, that it is also hidden behind a seemingly tedious puzzle, which I frankly don't see how anyone would figure out themselves naturally, is super unfortunate.
And I can't even come up with any good arguments in favor of this. One of the more famous 'complex' secrets in a rogue-like game is the Crystal ship in FTL, and even that one has dialogue hints and multiple ways to find out, and it only unlocks a new ship, not significant new content.
Same with a game like Binding of Isaac, while some of the very late game bosses require more complex puzzles these were only added later on, and the main "puzzles" are quite obvious and merely require collecting two pieces of a key. (And to beat the bosses a couple of times before)
All of that to say that I really dislike how the brain fight is presented. It makes you feel snubbed beating the heart and then suddenly not being allowed to go any further "just because". Better would have been if this secret puzzle remained mostly secret throughout, and was separate from the brain fight itself, could be the "tumor" or whatever, so it is kept more secret and doesn't snub the player from what is clearly the supposed ending.
normal atlas
edgy sinew