Staff of the Adder

From Dungeon Master's Guide, page 203. Found on Magic Item Table F.

Description
''Staff, uncommon (requires attunement by a cleric, druid, or warlock). 4 lbs.''

You can use a bonus action to speak this staff's command word and make the head of the staff become that of an animate poisonous snake for 1 minute. By using another bonus action to speak the command word again, you return the staff to its normal inanimate form.

You can make a melee attack using the snake head, which has a reach of 5 feet. Your proficiency bonus applies to the attack roll. On a hit, the target takes 1d6 piercing damage and must succeed on a DC 15 Constitution saving throw or take 3d6 poison damage.

The snake head can be attacked while it is animate. It has an Armor Class of 15 and 20 hit points. If the head drops to 0 hit points, the staff is destroyed. As long as it's not destroyed, the staff regains all lost hit points when it reverts to its inanimate form.