The Rotting Dark of Ufestra

You fill the air with a black wilting cloud of necrotic smoke and wind in a 5 foot radius, centred on a point within range. 

The caster of this spell has to have had means to the Book of Vile Darkness (or other dark avenues) to have learned it. 

Each creature in the spells  must make a constitution saving throw at the start of their turn. On a fail creatures take 6D8 necrotic damage and are cursed with "The Rotting Dark", on a success the creature takes half damage and are not cursed. To leave the cloud a creature must succeed on a strength (athletics) check against the spell caster's save DC or have their speed reduced to 0 for that turn. Cursed creatures have their CON score reduced by 1 at the end of their turn if they are still in the spell effect area. When the creature finally leaves the cloud the veins around their eyes, nose and mouth have turned black. Cursed creatures have their CON score reduced by 1D4 each day at dawn. The creature dies if their CON is reduced to 0. Otherwise, the reduction last until the curse is removed by: a remove curse spell, wish spell, or a miracle.