From Quake Wiki
Shub-Niggurath is the end boss of Quake.
The mighty Shub-Niggurath turns out to be an enormous wad of lard with tentacles. She can't even attack, but watch out -- she has a lot of Vores and Shamblers guarding her, and more will spawn in occasionally. To kill her, you need to telefrag her. See the spikey ball that floats around the level? See the teleporter at the end of the monster- and trap-infested hallway? When you step in the teleporter, you will warp to the location of the spikey ball. Do it when the spikey ball enters Shub-Niggurath, and you'll blow her to pieces. If you do it some other time, you'll most likely fall into the lava.
monster_oldone is the entity for placing Shub-Niggurath in a level.
|targetname||The targetname of Shub-Niggurath. When triggered, Shub-Niggurath will wake up if inactive.|
|target||The targetname of the entity to be triggered when Shub-Niggurath dies.|
|killtarget||The targetname of the entity to be removed when Shub-Niggurath dies.|
The monster_oldone entity supports the standard spawnflags for difficulty and deathmatch presence.
|256||Not on Easy||Shub-Niggurath will not spawn on Easy difficulty.|
|512||Not on Normal||Shub-Niggurath will not spawn on Normal difficulty.|
|1024||Not on Hard||Shub-Niggurath will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||Shub-Niggurath will not spawn in Deathmatch mode.|
This is the format used by TrenchBroom and Worldcraft. Note that the model(...) parameter is not supported by Worldcraft. This does not include the Monster base class definition, which is also required.
@PointClass base(Monster) size(-160 -128 -24, 160 128 256) model(":progs/oldone.mdl") = monster_oldone : "Shub-Niggurath" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_oldone (1 0 0) (-160 -128 -24) (160 128 256) */