Multiattack. The wyvern makes two attacks: one with its bite and one with its tail. While flying, it can use its claws in place of one other attack.
Bite. Melee Weapon Attack: +7 (+4) to hit, reach 10ft., one creature. Hit: 2d6+6 (1d8+4) piercing damage.
Claws. Melee Weapon Attack: +7 (+4) to hit, reach 5 ft., one target. Hit: 2d8+6 (2d6+4) slashing damage.
Tail. Melee Weapon Attack: +7 (+4) to hit, reach 15ft., one target. Hit: 2d8+6 (2d6+4) bludgeoning damage.
Breath Weapon (Recharge 5-6)
Cold Breath. The wyvern exhales an icy blast in a 30-foot cone. Each creature in that area must make a DC 15 (12) Constitution saving throw, taking 10d8 (5d8) cold damage on a failed save, or half as much damage on a successful one.