A 9th Level Ritual Crafted Xanos The God King Of New Thrace who has destroyed the instructions to perform it. 

The caster places the 5 onyx in a bowl then the caster spills their own blood until death onto the Onyx while keeping the target area of the spell in sight with it being the last image in their head as they die. The spell then activates, the target area being covered in magical darkness covering an area up to 2 square miles, no sound travels within the darkness nor can sound travel into the darkness. Everyone within the darkness has to make a constitution save, on fail they take a full 10d12 of Necrotic damage, if their health drops to 0 their bodies vanish and are brought back as shadows, on success they take half. Those still alive after the initial spell hit every 24 hours while they are still in the darkness they must make another constitution saving throw this time taking 9d10 necrotic damage and it lowers by 1 every day until the darkness fades. The Shadows created by those that died within are confined to the area of the spell for 100,000 years