Black Plague

You summon a rat. Every turn pick a spot for it to move to. In a line, connecting the points it moved between is a 1 foot wide black fog that deals necrotic damage to anyone who passes through it.

When you cast it you can choose people to not be affected by the fog. They will still be unable to see through it, but won’t be damaged by it.

The rat cannot attack or react. If it’s health drops to zero the spell is dispelled removing the fog. You can dispel the spell at any time on your turn for an action or bonus action.