Coldflame's Freezing Fire

User fires off a ball of light blue flame that is freezing to the touch. The flame is ice cold on contact but still burns whatever it comes into contact with. Creatures within the area of effect must make a dexterity saving throw, on a failed save they take 4d6 cold and 4d6 fire damage or half as much on a successful save. The creature(s) hit with this spell have their movement speed reduced by 10 feet.

At higher levels: 
For each level past 4 that this spell is cast add an additional 1d6 to both the fire and cold damages