The % Delirious stat in PoE 2 dictates the number and power of monsters that will attack you in the Delirium fog. The higher this number, the greater the number of enemies and the bigger the reward.

Breach, Delirium, Expedition, Essence, and Ritual return as mapping mechanics. Map bosses are now the exception, not the rule. Non-boss maps require you to kill every rare monster in the zone.