Lost soul

The Lost Soul is a flaming, flying skull first introduced in Doom's second episode, The Shores of Hell. Arguably one of the most frustrating enemies in the Doom games, Lost Souls float at a leisurely pace until they decide to attack, at which point they launch themselves towards the player at high speed. If they miss, they will continue careening through the air until they hit a wall or other object. The player can stop a lost soul's charge by shooting it while it is attacking. It is uncommon to find a lone lost soul on its own — they are more commonly found in groups. The Lost Soul is also the only monster in Doom that does not make any sound when it wakes up, sometimes surprising careless players.

Lost Souls do not leave a corpse - they simply explode. However, there exists a thing type named "Lost Soul Corpse". In the alpha versions of Doom, Lost Souls were grey, non-flaming and used a "psychic" attack, facing the victim and flashing for a moment, thus causing damage. These early Lost Souls left behind a pile of bones. As a consequence of having a 'corpse' sprite (albeit that it is empty), it is possible in rare circumstances for the Lost Soul to be revived by an Arch-Vile; specifically, if the Lost Soul is killed at exactly the same time it would be crushed by a door or crusher, it leaves behind the typical red 'splodge', which can be revived.

Doom II furthered the use of the Lost Soul by utilizing them as the projectile weapon of the new Pain Elemental monster.

In Doom II (and later in Ultimate Doom), Lost Souls were modified to no longer affect the player's kill score, so it is possible to achieve 100% kills without destroying all Lost Souls on a level. Some source ports have reverted to the older score system.

The Lost Soul is the only monster in Doom unaffected by the -respawn switch (including the Nightmare! skill level).