From Quake Wiki
Chthon is the end boss of the first episode, Dimension of the Doomed. You can find him in The House of Chthon, which is preceded by The Door to Chthon. He resides in a pool of lava, emerging once you pick up the episode's rune. He attacks by hurling explosive lava bombs (which, on Hard difficulty, are aimed ahead of moving targets). The lava bombs do the same amount of damage as a player's rocket (which is at least 100 upon a direct hit).
He is immune to player based weapons (via self.takedamage = DAMAGE_NO;), and can be damaged only by electrical prongs residing in the ceiling of his house, the code for which can be found in boss.qc. Switches are located near both of the prongs, and at the center-end of the level is a switch to activate them, electrocuting Chthon. On Easy skill you only need to do this once, but otherwise it will take three jolts. Upon death he sinks back into the lava, and a bridge forms over the sulfurous mass allowing you access to an exit from the arena.
"As the corpse of the monstrous entity Chthon sinks back into the lava whence it rose, you grip the Rune of Earth Magic tightly. Now that you have conquered the Dimension of the Doomed, realm of Earth Magic, you are ready to complete your task."
Chthon cannot be killed by regular weapons and must be killed by the lightning trap in e1m7 - the level in which he appears. To use the trap, the player must first press the buttons on the top level of the arena to lower the lightning rods, then press the button in the centre to fire lightning at Chthon. On Easy skill, a single hit is enough to kill Chthon, but on all other skill levels, three hits are required, which requires repeating the entire sequence of button presses for each subsequent attack.
Chthon's attacks are very easy to avoid on Easy and Normal skill, as he throws lava bombs directly at the player, so as long as the player keeps moving, he is unlikely to be hit. On Hard and Nightmare skill, however, the bombs are thrown in front of the player, so running without being careful is likely to end in the player's death. To successfully avoid the lava bombs on higher skill levels requires much more care.
monster_boss is the entity for placing Chthon in a level.
|targetname||The targetname of Chthon. When triggered, Chthon will wake up if inactive.|
|target||The targetname of the entity to be triggered when Chthon dies.|
|killtarget||The targetname of the entity to be removed when Chthon dies.|
monster_boss does not support the ambush spawnflag that is supported by regular monsters. Instead, it must be triggered for Chthon to appear at all.
It does, however, support the standard spawnflags for difficulty and deathmatch presence.
|256||Not on Easy||Chthon will not spawn on Easy difficulty.|
|512||Not on Normal||Chthon will not spawn on Normal difficulty.|
|1024||Not on Hard||Chthon will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||Chthon 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(-16 -16 -24, 16 16 40) model(":progs/boss.mdl") = monster_boss : "Chthon" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_boss (1 0 0) (-128 -128 -24) (128 128 256) */