Agnevari's Imploding Ring of Flames

You create a ringed wall  of fire 20 feet in diameter, 10 feet high, and 1 foot thick. The wall is opaque and lasts for the duration 

When the wall appears, each creature within 5 feet of it must make a Dexterity saving throw. On a failed save, a creature takes 3d8 fire damage, or half as much damage on a successful save. This damage also applies to any creature passing through the wall.

At the end of the duration, on the beginning of your turn the ring implodes inward in a massive cascade of flames. Any creature within its area must make a Dexterity saving throw. On a failed save, a creature takes 10d8 fire damage, or half as much damage on a successful save.

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the damage increases by 1d8 for the implosion damage only for each slot level above 4th.