Vahn's Voracious Vertebrate

You use your action and point at an area on the ground within range. A bony mouth erupts from the ground in a 10ft wide cylinder and bites all creatures in the area up to a height of 30ft. Creatures in the area make a Dexterity saving throw. Creatures that fail take 6d10 necrotic damage and you gain temporary hp equal to half the damage dealt for 1 minute. Creatures that save take half that damage and the caster does not gain temporary hp.

If the caster spends 10 minutes casting this spell within the boundary of a magic circle the mouth is instead summoned as a flameskull for 1 day. The flameskull is initially under your control but after it takes any damage it can make an Intelligence saving throw against your spell save DC. On a success the flameskull breaks free and becomes hostile towards you. If the flameskull is reduced to 0 hp it automatically succeeds this saving throw when it regains 1 hp.