Deadly Sting

You grow a 10-foot-long tails as supple as a whip, tipped with a horrible stinger. During the spell's duration, you can use the stinger to make a melee spell attack with a reach of 10 feet. On a hit, the target takes 1d4 piercing damage + 4d10 poison damage, and a creature must make a successful Constitution saving throw or become vulnerable to poison damage for the duration of the spell.