Yeah, I think Rune's ending was supposed to be a slap on the wrist/bad end for "too greedy" players, but it's not really balanced properly as it is now.
Disregarding the gallery spilling the beans on how to get it or looking it up outside of the game, the only way for it to happen "organically" is for the player to go all in on the tentacle rooms with the intent of selling at least 100 eggs for gold, while still having lots of them left over to get even more essence for brainwashing all the NPCs before the time is up. Issue is, money quickly becomes absolutely worthless and you need max affection to brainwash each NPC, so you still need to get all the "affection milestones", which means playing the game properly, which means making a ton of money with no means of spending it, which means you will just stop bothering to sell the eggs before you even get to a 100.
I think it'd work better if brainwashing had a lower affection requirement (or none at all, but higher essence cost?), therefore making a "Rune speedrun" (speedrune?) possible (ignore the story and affection, tentacles only, brainwash all, have 50k or more by day 50, get the bad end).