When Entrophy seeps into a metal vein or through cracks deep into stone and corrupts it, it turns into Blackstone. It appears as some sort of petrified type effect. Weakened and corrupted by the Entrophy, it begins to corrupt underground waterways and weaken stone all around it. It is used to make blasphemous and evil altars, containers for dark souls, a curtain against the light, and other dark magics. Once the scent of it makes its way into Underlight Realms (like The Deeping, etc.) or above ground it draws the Underlight denizens, Orrish raiders, fell creatures of the shadows, evil magi and priests and others of their ilk.
Ducateon make a habit of rooting out and destroying Blackstone. It is mined, crushed, powdered and scattered (even thrown into lava when it can be), and used in quench ponds to create black weapons (it absorbs the darkness – no other effect). The Ducateons see it as the ultimate essence of chaos and their orderly nature drives them to destroy it. The process is always overseen by the priests of Him.