Chthon is one of the three "final" bosses of Quake - being the demon faced at the end of the first stage: it is a gigantic monster that resides in a lake of magma and attacks via hurling balls of fire at the player.

Chthon is the ruler of the "Doomed Dimension" and presumably controls the varied monsters found in the area - all of which are hostile towards the player and will attack on sight.

Due to being the first boss of the game he is rather simplistic and should not pose too much of a threat - somewhat ironically the journey to confronting Chthon is often more dangerous than actually facing the demon.