Blood

Blood is ubiquitous in the Doom games, perhaps a large factor in making them controversial in some circles (and appealing in others).

In many levels, blood flows in rivers, over blood-falls, and gathers into pools. Standing in or moving through blood is usually detrimental to a player's health (see damaging floor).

The Doom rendering engine creates animated blood spatter when a "shootable" Thing is hit (including the player!). Besides enhancing the atmosphere of the game, this helps a player to know that his or her aim was correct. (For realism, inanimate objects, including barrels, produce a puff of smoke rather than blood spatter when hit.)