In Quake, Chthon is the final boss at the end of the First Episode (and first boss out of two in the game). It's a large Monster that lives in the pool of Lava that centers the House of Chthon. Chthon's face consists of a vertical mouth with jagged teeth, no eyes, and no nose.
Chthon's only attack is to pick up balls of molten rock from his pool and throw them at the player. These balls deal a considerable amount of damage of between 100 to 120 for a direct hit, on top of having a massive splash damage radius, so getting hit but once would be enough to kill a player who doesn't have any type of Armor or the 100 Health (Easy and Normal only). On harder difficulties he will frequently lead his shots, easily killing the player if they aren't paying attention.
Chthon cannot be hurt by the player's normal weapons due to his impregnable armor. However, there is a trap set in the ceiling of the House of Chthon where the player does battle with the Chthon. To defeat this boss, the player must ride to the first floor and activate the two Buttons on either side of the House. This lowers two Electric Terminals on either side of the Chthon. Then the floor panel at the far end can be used to activate the Electric Terminals, zapping the Chthon and damaging him. On Easy this need be done only once to kill him, while on Normal, Hard and Nightmare 3 hits are needed to sink this demon back into the pit of Lava whence it rose. A Rocket Launcher can help you a bit, as you can Rocket Jump to help save some time.
"Player" was incinerated by the Chthon (Cut from game)
Chthon have a relation to the Greek word chthonios, "in, under, or beneath the earth." Being made of lava, this definition seems plausible. It could also be in relation to the Greek word "kthon," meaning "dweller under the earth."
"Chthon" literally translates to "Earth" in ancient Greek, and Chthon guards the Rune of Earth Magic.
Although there is a death message written for Chthon in client.qc (seen above), he has no actual way of directly killing the player, thus the message is never seen in-game.
While Chthon only has 3 Health, it cannot be damaged in any way by conventional Weapons. If the player is somehow able to injure Chthon (such as with modifications), he will not be injured, but his AI will break and he will be unable to attack the player until he is injured by an Electric Terminal.