《SPOILERS 》There is some idea that if conceived can destroy you. Greco wants to use it to attack the protagonist. The protagonist defends himself by delaying his perception and passing all inputs through a simulated concept so that when Greco communicates the idea it would destroy the sandbox and not himself. But Greco never tells the protagonist the idea. It just lets the protagonist formulate it themselves as a reaction to all the triggers Greco planted beforehand. Before in the story the prot limits his mind not to think "dangerous" things but the limitation of that model is that you need to have a way to describe what not to think about without thinking about them actively. Since hes not aware of the idea existing beforehand he has no limits in place against that. And so they lose their mind.
I had a slightly different take that there was no highly advanced "attack". Greco merely encouraged the protagonist to drop their ego boundary, causing the protagonist self-centered mental breakdown to occur. It seemed clear to me that throughout the story the main form of the protagonist unhappiness and stress stemmed from an egocentric orientation (total separation from society and nature).
