Aquamarine Dagger

Upon first glance, this weapon appears to be a rusty dagger, with a chunk of the blade chipped off to the point of almost being described as a hilt.

Once the user has either attuned to the weapon, or killed a monster of great power with the dagger on their person, the innate arcane power of the dagger will be released.

As a bonus action, the user can twirl the dagger to release the stored magic of the countless aquamarine crystals, creating a fleeting blue crystal blade, lengthy enough to swing as a shortsword. The weapon can stay in this crystalline form for up to 10 minutes. Once this property of the dagger has been used, it cannot be used again until the next dawn.

When the weapon has been transformed into the Aquamarine Crystal version, the creature gains the following benefits:

When you hit with an attack using this magic sword, the target takes an extra 1d6 cold damage + 2 piercing damage. In addition, while you hold the Aquamarine sword, you have resistance to fire damage. In freezing temperatures, the blade sheds bright light in a 10-foot radius and dim light for an additional 10 feet. 

When you draw this weapon, you can extinguish all non-magical flames within 30 feet of you. This property can be used no more than once per hour.

Proficiency with a Dagger allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage: Cold, Damage: Piercing, Resistance: Fire, creature with magical potential, Finesse, Light, Thrown, Nick