Carbon Spear, AKA Speed of Dark.

Weapon (Spear), very rare (requires attunement)

A long time ago, on a mountaintop far, far away, a mighty storm let loose elemental forces, and in its fury, lightning nearly blew the top off the mountain. After the slag had cooled, all that remained of the sacred tree that had once stood there was a charred branch that had been transformed into superconducting carbon.

An Elven mage, investigating the blast, found the brach. Bringing it back to his workshop, he reinforced it with runes of power and topped it with highly conductive meteor metal which he forged into a spearhead. Thus the Carbon Spear was born.

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The spear is a magical +2 to hit and damage weapon. On a hit from a normal attack, it deals an additional 1d8 lightning damage.

Immediately after hitting or missing, the weapon flies back to your hand.

The spear has a maximum of 6 charges. The spear regains 1d6 charges after a long rest. If there is a storm during the long rest, the spear automatically fills to its maximum number of charges.

Lightning Arrow Effect

When the spear is thrown, you can spend one charge to have it act as per the Lightning Arrow spell:

The weapon transforms into a bolt of lightning. Make the attack roll as normal. The target takes 4d8 lightning damage on a hit, or half as much damage on a miss, instead of the weapon’s normal damage.

Whether you hit or miss, each creature within 10 feet of the target must make a Dexterity saving throw (DC 10 + your Dexterity bonus). Each creature takes 2d8 lightning damage on a failed save, or half as much damage on a successful one.

The spear returns to its normal form and immediately returns to your hand.

Teleport effect

When the spear is thrown, you can spend two charges to teleport to wherever the spear hits. You must roll to hit the target. Normal spear throwing rules and distances apply.

On a hit, you are transported in a flash of lightning and a clap of thunder to where the spear hits.

When transported, the flash of lightning briefly illuminates the path of the spear in bright light to 30 feet and dim light to another 30 feet. The thunderclap can be heard up to 100 feet away.