Antiflame Burst

Your trace a fell rune in the air, leaving a trail of cool blue fire where your fingers pass. The runes wink out and a small locus of blue flame shoots out to a place you can see within range; the flame winks out for a moment before bursting forth in a torrent of blue and black fire. Creatures in a 15 foot radius must make a Constitution saving throw or take 5d4 cold damage + 5d4 necrotic damage,  and have their movement speed halved for the next round. On a successful save the creature takes half damage and suffers no decrease in speed.

At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the cold and the necrotic damage increase by 1d4 per slot level over 3rd.