D&D 5th Edition: Frostrazor, A Magic Longsword

"A nameless archmage crafted the blade by impaling an ancient wyrm of a white dragon with an enormous icicle stalactite, knocking it from the roof and impaling the beast through the heart before using its blood and magic to shape the icicle into a massive, crystaline blade of never-melting ice. He did this to create a superior weapon that could be used in his war against an efreet pasha. Unfortunately for him, his trusted general was actually working for the pasha, and promptly stabbed him in the back with the newly forged longsword."

FROSTRAZOR

Magic weapon (longsword), legendary (requires attunement)

• You gain a +3 on attack rolls and damage rolls with this longsword.

• Frostrazor deals 1d10 cold damage and has the Finesse property.

• When you score a critical hit with Frostrazor, a 6 inch long shard of ice breaks off in the target's wound. A creature wounded in this way takes 1d8 cold damage at the beginning of their turn and you gain that many temporary hit points. They may use their action to make a DC 14 Constitution saving throw to dig the shard out of their wound wounded in this way, 

• When you kill a creature with Frostrazor it turns to ice.

• You gain resistance to cold damage and do not gain levels of exhaustion from freezing temperatures.