From the introduction:
None know from where the Heresiarch first came, but all remember the night that it did.
It rode down from the bleeding stars on a great serpent, hurling bolts of obsidian lightning that shattered the monuments and capitols of every nation. Its infernal army swept aside the defenses of the mortal empires in a single hour, decimating legions once thought to be the invincible fist of humanity's god-kings.
Faceless priests - each bearing the symbol of the trident - drifted through the fallen cities and scorched villages on a frigid wind, and when they rose to greet the huddled men and women ringed by their festering, bloated dead, they spoke a single, simple offer: worship the Heresiarch or die.
Thousands of crusaders fell tonight so that you might be given this chance.
In a last stand that, for the first time, united all of the empires of humanity as brothers and sisters, a way was cleared into an infernal stronghold said to contain a gate to the Heresiarch’s fane.
All is silent save for the clangor of distant battle. Surrounded by grim-faced knights and teary-eyed peasants – their hands clasped in desperate hope – you step through the glowing, churning doorway, knowing there will be no help and likely no return.