Corrupted Wyvern

Unnatural Flight. Despite its bony wing structure, the corrupted wyvern can still fly.

Corrupted Fortitude. If the corrupted wyvern is reduced to 0 hit points, it can make a constitution saving throw of 5+ the damage taken, if it succeeds, it is instead dropped to 1 hit point.

Corruptor. If the corrupted wyvern rolls a critical hit on its stinger attack or bite attack, the targeted creature must make a DC 18 constitution saving throw or become corrupted. If untreated for a week, the creature will transform into a corrupted or strayed creature, 2 days afterwards they will stop at nothing to return to where they were corrupted.

This can be cured with a Greater Restoration spell.

Horrific Visage. All creatures that start their turn at least 30 feet from the corrupted wyvern have to make a DC 20 wisdom saving throw or become frightened until the start of their next turn.

Multiattack. The Corrupted wyvern either makes 2 claw attacks and 1 bite attack or 1 claws and 1 stinger attack.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 19 (4d6 + 5) piercing damage plus 8 (2d8) necrotic damage.

Claws. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 16 (2d12 + 4) slashing damage. 

Stinger. Melee Weapon Attack: +12 to hit, reach 15 ft., 1 target. Hit: 13 (2d10 + 3) piercing damage plus 19 (3d10) necrotic damage, the targeted creature has to make a DC 19 constitution saving throw or become poisoned. The creature also has to make a DC 17 strength saving throw or become grappled and restrained. While restrained in this way, the creature is pulled into the ribcage of the wyvern and it cannot use its stinger attack until it opens its ribcage again.