Started with the birth of Hyborem and the Infernal civilization, Hell Terrain will spread as allowed by the Armageddon Counter (AC). It will always spread into Ashen Veil lands, but if the AC is over 25 it will also spread into unowned lands. If the AC is over 50 it can spread into other Evil lands and if it is over 75 it can spread into Neutral lands. It can never spread into the lands of Good players.
Hell Terrain is less productive than most land and the bonus types are often replaced with Hell versions. For example Cows or Horses on a normal terrain type will be converted to Nightmares if the tile switches to Hell Terrain.