Baldart's Creeping Inferno

The caster uses the spell at their feet. They must hold concentration to keep create and contain a short circle of around them. Each turn, they can use their bonus action to spread the fire outward by up to 10 ft per turn. The fire will only damage those the caster deems an enemy; though the mini-flames barely seem a threat. Anyone the flame barrier passes through or to creatures that pass through it will take damage and require a CON save. 3d6 fire and 3d6 necrotic damage will be dealt to the enemies as the thin line of fire rises up and engulfs them as a net of necrotic fire, immobilizing them until the end of their next turn. A successful save will mean half damage and will not be immobilized. 

This damage will occur anytime the creature enters or leaves the circle of fire. (Does not apply to those flying over 10 ft from the ground). 

additional 1d6 fire and necrotic damage per level above 6th. 

The flames can continue to grow or shrink by up to 10 ft per turn (only one direction at a time, must use a turn to stop / reverse them). When the fires die or concentration is broken, the caster must succeed an INT throw of 5 or take 10 necrotic damage.