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.