Belfort's Ring of Smoke

You create a runic ring of caustic smoke10 feet in diameter, in which a creature takes 2d6 Fire Damage if it enters or passes through.

If a creature starts or ends its turn in the ring of smoke, the creature must make a Constitution saving throw. On a failed save, the creature takes 6d10 Fire damage. On a successful save, the creature takes half as much damage.  Attacks or visual Checks are at Disadvantage while within the ring.

At higher levels, you create up to 3 rings on the battlefield.