As an action, you create a shard of ice and fling it at a creature within 10 meters. Make a ranged spell attack against the creature.
On a hit, its takes 2d8 piercing damage.
If you miss by 4 or less, it takes half as much damage.
Hit or miss, the shard then explodes. The target and each creature within 1 meter of it must make a Reflex saving throw. A creature takes 2d6 cold damage on a failed save, or half as much damage on a successful one.
If the knife strikes a body of water or a liquid that is principally water (not including water-based creatures), it freezes the liquid to a depth of 15 centimeters over an area 1 meter in radius. The ice is an instantaneous effect, but persists as nonmagical ice. Under temperate conditions, the ice lasts 1 minute. Creatures that were swimming on the surface of frozen water are trapped in the ice. A trapped creature can use an action to make an Acrobatics or Brawn check (its choice) against your spell save Difficulty to break free.
You can increase the piercing damage by 2d8 and the cold damage by 2d6 for each additional mana expended.