Lost soul limit

From Doom Wiki

To prevent overloading the engine with lost souls, a hard-coded lost soul limit was added which prevents pain elementals from spawning them if there are already 21 lost souls in the level. Because pain elementals have no other attacks, this has the side effect of rendering them helpless when the limit is reached.

This limit is removed in some source ports, although various engines allow the user to toggle the limit because some levels with a good number of pain elementals may become overpopulated with lost souls, especially when playing with the fast monsters setting enabled.

Hell Revealed's MAP19: Everything Dies utilizes the limit to create a trap at the beginning of the level. The player starts inside a room with a megasphere in the middle, and four pain elementals, rendered harmless by the presence of three dozen lost souls in an inaccessible room. Grabbing the megasphere, however, will cause all of the lost souls to be crushed, allowing the pain elementals to attack the player.