Weapon (whip), rare
When you roll a 20 on your attack roll with this magic weapon, the target takes an extra 7 damage of the weapon’s type.
Proficiency with a whip allows you to add your proficiency bonus to the attack roll for any attack you make with it.
Notes: Damage: Additional, Damage, Combat, Finesse, Reach
As a DM, I would allow any critical hit with this weapon grants the extra damage.