Diamondice Dagger

Weapon (dagger), very rare (requires attunement)

This slim stiletto has a diamond-hard ice blade and is a +1 dagger that also deals 1d4 cold damage on each hit (2d4 on a critical hit). The blade is as hard and durable as adamantine, though its blade is of crystal rather than metal. A diamondice dagger deals no damage to objects or creatures made of ice or snow. When you score a critical hit against a creature with resistance or immunity to cold, the target must make a DC 15 Constitution save or lose that resistance or immunity for 1 round. If the resistance or immunity is provided by a spell, that effect is dispelled.

Once per day, you may thrust a diamondice dagger into a snowy or icy surface, filling either a 15-foot-long line or a 10-foot cone-shaped burst that area with razor-sharp shards of ice. This area is difficult terrain, and creatures must make a DC 14 Dexterity save each time they enter a square of ice shards or take 1d4 cold damage and 1d4 piercing damage. If you leave the diamondice dagger planted in the ground, the length of the line increases by 15 feet or the length of the cone increases by 5 feet, up to a maximum a 60-foot line or a 30-foot cone. A 5-foot square of icy shards can be destroyed by dealing at least 5 points of fire damage or 20 points of bludgeoning damage.