Cosmic Comet

Invoke the chaotic nature of the very cosmos itself in order to summon a devastating comet to demolish your enemies. Upon casting of this spell, an opaque portal appears 50ft above any chosen point within range. A shimmering, purple comet of pure magic is then ejected from the portal at a high velocity, exploding in a 5ft radius sphere upon impact.

Any creature caught within the radius of the explosion must make a Dexterity Saving Throw, suffering 1d100 + 10 Force Damage on a failed save, or half as much damage on success. Rolling a natural 100 for the damage causes any affected targets to instantly perish, whether the saving throw was successful or not, as their very being is ultimately erased from the fabric of reality. Whether this bypasses legendary resistance or not is up to DM discretion.
(Since there is no d100 option for damage, roll two d10's and treat the results as if they were a single number: 6 + 4 = 64, rather than 10)

Casting this spell puts such an enormous strain on the mind that users must spend their subsequent turn recovering, costing an additional action.
If used on a target with less than <50ft of free space overhead, this spell has no effect. 

Using a Higher-Level Spell Slot. The bonus damage modifier increases by +10 for each spell slot level above 5. (i.e: A 7th level Cosmic Comet would deal 1d100 + 30 Force Damage)