Tag 666

Tag 666 is a special tag number used to denote map-specific behavior. Tag 666 is invoked not by crossing a linedef, but by killing monsters. The specific behavior is defined in p_enemy.c.

Relevant maps
Tag 666 can be used in E1M8 of Doom; E4M6 and E4M8 of Ultimate Doom; MAP07 and MAP32 of Doom II; and E1M8, E2M8, E3M8, E4M8 and E5M8 of Heretic.

Tag 666 can also be used in any Doom II WAD that contains Commander Keen figures, no matter which level they are on. Once all the Keens on a level have been killed, all sectors tagged 666 will open as doors.

In Strife, tag 666 may be used in any map that contains a Spectre "E", which is the spectre spawned at the death of the Loremaster. When the spectre dies, all sectors tagged 666 will lower to the lowest surrounding floor.

void A_KeenDie(mobj_t* mo)
This procedure is called by the death of each Commander Keen present in the level. When the last Keen dies, doors tagged 666 are opened. This procedure was used as part of an easter egg in MAP32 of Doom II. Note that this routine checks for all things of the same type as the calling monster, rather than checking for the  type directly, so it can easily be given to other monster types via DeHackEd.

void A_BossDeath(mobj_t *mo)
This procedure is called by the death of a boss monster. However, it only takes effect if the monster is the last of its type on the level. There are additional restrictions which differ depending on the game and level.

Doom

 * E1M8
 * Executes  when all Barons of Hell are dead.


 * E2M8
 * No sector action: level ends when all Cyberdemons are dead.


 * E3M8
 * No sector action: level ends when all Spider Masterminds are dead.


 * E4M6
 * Executes  when all Cyberdemons are dead.


 * E4M8
 * Executes  when all Spider Masterminds are dead.

Doom 2

 * MAP07
 * Executes  when all Mancubi are dead.
 * Executes  on sectors tagged 667 when all Arachnotrons are dead.

Heretic

 * E1M8
 * Executes  when all Iron Liches are dead.


 * E2M8
 * Executes  when all Maulotaurs are dead and kills all remaining monsters (by calling P_Massacre).


 * E3M8
 * Executes  when D'Sparil is dead and kills all remaining monsters (redundantly since D'Sparil's death does the same thing, A_Sor2DthInit calls P_Massacre before A_BossDeath is run).


 * E4M8
 * Executes  when all Iron Lichs are dead and kills all remaining monsters.


 * E5M8
 * Executes  when all Maulotaurs are dead and kills all remaining monsters.

Strife

 * Any Map
 * Executes  when all Spectre E are dead and at least one player is currently alive.

Difference in behaviour between Doom1 and Ultimate Doom
The behavior of tag 666 changed with the release of Ultimate Doom. A constraint was added which restricted tag 666 on E1M8 to triggering only on the death of the last Baron, rather than the death of any last boss monster. The classic PWAD Doomsday of UAC relied upon the deaths of both the last Baron and the last Cyberdemon to trigger it, making the PWAD unplayable in most modern source ports. The original behavior is supported in Chocolate Doom, Doomsday and PrBoom through optional compatibility options, and in ZDoom, by autodetection of the level.

Specifically, the following code is responsible (A_BossDeath in p_enemy.c):

switch(gameepisode) {       case 1: if (gamemap != 8) return; if (mo->type != MT_BRUISER) return;  break;

The code in bold type defines the new constraint.