Slither Sling (5e Equipment)
From D&D Wiki
Weapon (sling), legendary (requires attunement)
This sling takes the form of a living snake, which coils itself around the upper arm of its owner. When needed, the owner can transform the snake into a sling at will. On a successful attack roll, any projectile fired by the sling deals an additional 1d8 poison damage.
Free Snake. This snake rolls its own initiative, attacks on its own turn, and understands Common. It has the ability scores of a poisonous snake except for an Intelligence score of 7 and the snake instead has 1/2 of your maximum. If killed, the snake vanishes and reappears coiled on your arm after 24 hours.
Polymorph. The snake can use an action to cast polymorph on itself, allowing it to transform into any snake type creature. The maximum cr of the snake is determined by your health. For every 10 hp the maximum cr increases by 1/2. This last untill dispelled or loss of hp. The snake regains use of this effect after finishing a long rest.