Dark miasma

A portal opens up in the air or on the ground and a black miasma pours forth.

You create a 20ft radius circle of black miasma on the floor. Each creature inside standing in the miasma must make a Constitution saving throw. On a success they take half damage, on a fail they take 2d6 necrotic damage and their speed is reduced by 10ft.

For every level above 2, this spell gains 1d6 of damage for every 2 levels the target's speed is reduced by a further 10ft. If this spell reduces a target's speed to negative they are pulled to the ground and considered prone.