Maps containing a Delirium Mirror can have the mirror walked through to unleash the rolling Delirium fog ring across the current area.
You must stay within this fog as it expands to maintain the Delirium or it will disappear.

Defeating any monster while in this fog adds to a rewards counter, with the culminated rewards being dropped after the fog dissapears.
Simulacrum Splinters can be collected from Delirium Monsters to form a Simulacrum that allows access to a challenging Delirium encounter via the Realmgate.

Magic monsters are able to gain Delirium specific Modifiers while inside the fog.

Rare or Unique monsters in the fog may manifest Delirium Demons. Manifested Delirium Demons inhabit the monster's body to occasionally use Skills of their own.