The Mire (MD)

As an action, you summon a portal that creates a slowing field of eldritch mire at the target location in a  30-foot-radius, 100-foot-high cylinder centered on a point within range. A creature moving through the area must spend 4 feet of movement for every 1 foot it moves.

As an action on your second turn, The Mire's radius increases by 5 ft, and a large mass of corruption drops from the portal, each creature within the radius needs to make a Dexterity saving throw or take 4d6 necrotic damage and have their movement reduced to 0 on a failed save, or take half as much damage and suffer no further effects on a successful save.

As an action on your third turn, The Mire's radius increases by an additional 5 ft to a final radius of 40 ft, as a large mass of corruption drops from the portal, each creature within the radius needs to make a Dexterity saving throw or take 4d6 necrotic and be stunned until the end of their next turn on a failed save, or take half as much damage and suffer no further effects on a successful save.

Not using your action on subsequent turns stops The Mire from growing and firing additional shots. After the spell ends, the area remains for an additional round