Untrained Sting. The wyvern is not good at restaining its injection of venom during a sting. The wyvern begins each day with three charges. When the wyvern hits with a stinger attack, it expends a charge. If there are no charges left when the wyvern hits with a stinger attack, the attack does not deal any poison damage.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 3) piercing damage.

Stinger. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) piercing damage, and the target must make on a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save or half as much damage on a successful one.