Python Bladewhip

Trick weapon (whip, rapier), very rare (requires attunement)
You have a +2 to attack and damage rolls made with both forms of this weapon.
Attunement. When you attune to this weapon, the cold, lifeless snake wraps around your forearm and bites your flesh, burying its head beneath it and inflicting 1d4 piercing damage. While attuned, you cannot be disarmed of this magical weapon. Ending the attunement causes the head to retract and the item to fall to the ground.
Trick Weapon. As a bonus action, you can switch between the transformed and untransformed states of this weapon. In its untransformed state, the bladewhip is a metallic whip. In its transformed state, the weapon solidifies by coagulating your blood to form a rapier.
Untransformed. Attacks you make with the python bladewhip add an additional 5 feet to your reach, as well as when determining your reach for opportunity attacks with it. In addition, if you hit a creature twice in a turn with this weapon, the whip's devouring hunger awakens and tears out more of its flesh, dealing an extra 4d4 slashing damage, and at the start of the target’s next turn, it takes an extra 2d4 slashing damage.
Transformed. While transformed, you take 1 piercing damage at the start of each of your turns as the blade drains your blood to empower itself. In addition, you gain immunity to the poisoned condition and to curses from spells (such as from bestow curse). This rapier gains the Fireblood Poison property and is considered silvered.
Fireblood Poison. Once on each of your turns when you hit a creature with an attack using this blade, you can coat the wound with poison and the target must make a DC 16 Constitution saving throw. On a failure, it is poisoned for the next minute as its innards start to boil. This can affect creatures immune to the poisoned condition. While poisoned in this way, the creature takes 3d6 fire damage at the start of each of its turns. This damage ignores resistance to fire damage.
A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the blade's Fireblood Poison for the next 24 hours.