Venomous Sword

Sword, any (requires attunement)

This magical sword deals an extra 1d6 poison damage.

Whenever the sword deals poison damage to a creature, you can use your reaction to make the blade of the sword become an animate poisonous snake.

You can make a melee attack using the snake that the blade has become, which has a reach of 5 feet. On a hit, the target takes 1d6 piercing damage and must make a DC 16 Constitution saving throw. On a failure the target takes 4d6 poison damage, or half as much on a success. This damage replaces the weapon's original damage and poison damage.

While the blade is animate and no other hostile creature is within 10 feet of the blade, you lose control of it and the snake attacks you on its own during the start of each of your turns using your proficiency bonus. The snake is out of control and keeps attacking you until you reach 0 hit points, at which point it reverts back to its inanimate form. As long as you control it you can use a bonus action to revert the blade to its inanimate form.

The sword's blade can be attacked while it is animate as a snake. It has an Armor Class of 16 and 30 hit points. If the blade drops to 0 hit points, the sword reverts to its normal inanimate form.