During in-map Delirium events in high level Waystones, you will be rewarded with Simulacrum Splinters. 50, 100 or 150 of these allow you to access the Simulacrum, which is done in the Realmgate on the Atlas. Using larger numbers of splinters makes the fights considerably harder, adding 20% delirium to each wave, pushing it to 115% in the final wave of the 150 splinter version.
The Simulacrum is a series of 15 combat encounters that take place in a replica of a POE2 town. Each fight is typically harder than the last.
After each encounter you will receive a number of Distilled Emotions based upon the number and power of the monsters you killed. From 0.3.0 onward, these appear to bias toward rarer Distilled Emotions in the 150 splinter version.
Upon completing all 15, you will receive an additional unique item exclusive to the Simulacrum.
All Simulacrum uniques can be found in all difficulties, but higher difficulties very aggressively bias the drops toward certain items.