From Quake Wiki

The item_sigil is the Rune located at the end of each of the four episodes of Vanilla Quake. These Runes are persistent in that once one is collected they will stay with the player throughout the rest of the game (unlike other pickups such as weapons, ammo or armor that will reset if the player enters a level named "Start"). It serves three purposes.

  1. To indicate to the player that they have reached a goal in the game.
  2. To trigger a func_episodegate in the start level that is linked to the Rune.
  3. To trigger a func_bossgate in the start level once all four Runes have been collected.

The last two functions are only available after the player exits a level through a (trigger_changelevel) and is sent to another level.

Using an item_sigil in a map[edit]

In an .fgd file there is usually this entry for the item_sigil:

@PointClass size(-16 -16 -24, 16 16 32) base(Item, Appearflags) = 
	item_sigil : "Sigil" 
	spawnflags(Flags) =
		1 : "Episode 1" : 1
		2 : "Episode 2" : 0
		4 : "Episode 3" : 0
		8 : "Episode 4" : 0

This entry tells a level editor (such as Worldcraft) that it has the options of being selectively available in easy, medium, hard, nightmare or deathmatch skill levels (Appearflags) and then it gives the choice of which rune of the four runes to insert into the level (the default in this case is the item_sigil for episode 1 as indicated by a 1 instead of a 0).

It is entirely possible to have all four runes inside of one map which can lead the level designer into some interesting decisions later on in a follow-up level.