Level event

Each level (expect for arena type levels) starting with dlevel 10 has a chance of having a level event. The chance of rolling for a level event is min((dlevel / 2) - 1, 19) out of 100. (It is possible to roll for no event even if this check succeeds.)

The type of event is selected by a roll of 1d10 + difficulty. Each event has an associated level feeling.

Deadly Air Events

The rate of health depletion is every 0.1s × (100 - 5 × difficulty).

Armed Nuke Events

The countdown starts at (10 - difficulty) minutes.

Flood Events

The flood type can be either acid or lava. On Hurt Me Plenty and easier difficulties, acid is always used. On harder difficulties, lava is used once dlevel is at least 17.

The flood rate is usually 0.1s × max(200 - dlevel - 5 × difficulty, 60) per column. However, on Ultra-Violence and Nightmare! and if dlevel is at least 21, there is a 1/5 chance for the flood rate to be set to a ridiculous 2.5s per column.

Flood event levels feature special player and stairs placement. If possible, the player will almost always start with an x coordinate less than 20, and the stairs will have an x coordinate greater than 60. (If one of these is not possible, or very rarely on other occasions, the position will simply be random as usual.)

Frozen Events

All level walls can be destroyed by any damage type, and all fluids that spawn on the level are water.

Sturdy Events

All level walls are indestructible.

Alarm Events

All enemies on the level hunt the player precisely, similar to bruiser brother enemies and other boss types (as a technical note, all beings are given the BF_HUNTING flag). Note that a particular being's AI still determines how accurate their path-finding is in tracking the player down.