Magic Items

It uses the stat block of a giant poisonous snake.
The serpent is a construct that understands and obeys whoever was holding the staff when it transformed.
It has immunity to poison damage and the poisoned condition. It doesn’t require air, food, drink, or sleep.
When it drops to 0 hit points, roll a d6. On a roll of 1, the snake turns to dust and is destroyed. On any other roll, it changes back into a staff and breaks into 1d4 + 1 pieces that must be magically mended before the staff can be used again.