Bone Shadow Wyvern

Devil's Sight. Magical darkness doesn't impede the Wyvern's darkvision. 

Drag & Drop. If the wyvern flies at least 30ft straight toward a target and then hits it with a Claw attack on the same turn, a large or smaller creature is grappled(escape DC 14). Until this grapple ends, the target Restrained, and the Bone Shadow Wyvern can't grapple another target. While grappling a target, the wyvern can only ascend 30ft while flying, and it can choose to end the grapple and drop the target anytime during subsequent turns. The target takes 3(1d6) bludgeoning damage for every 5ft it descends and is knocked Prone. If the target hits a creature, the creature must make a DC 16 Dexterity saving throw. On a failure, the creature takes the same amount of damage the target takes and is also knocked Prone. On a successful save, the creature moves to the nearest unoccupied space within 5ft of it. If there is no unoccupied space within 5ft of it, the creature automatically fails this saving throw. 

Fire Incarnate. The Vengeful Spirit's influence gives the Bone Shadow Wyvern the ability to engulf its body in violent hellfire, as the spirit itself can. While this fire rages, all attack made by the Bone Shadow Wyvern deal an additional 5(2d4) fire damage. This fire damage ignores resistance to fire damage.

Flyby. The wyvern doesn't provoke opportunity attacks when it flies out of an enemy's reach. 

Multiattack: The Bone Shadow Wyvern makes three attacks, one with its Bite and two with its Claws. 

Bite: Melee weapon attack, +9 to hit, reach 10ft, one target. Hit 16(2d10+5) piercing damage plus 5(2d4) fire damage. 

Claw: Melee weapon attack, +9 to hit, reach 10ft, one target. Hit 12(2d6+5) slashing damage plus 5(2d4) fire damage. 

Fire Breath(Recharge 5-6): The wyvern exhales fire in a 60ft line that is 5ft wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 55(10d10) fire damage on a failed save, and half as much damage on a successful one. 

Shadow Breath(Recharge 5-6): The wyvern exhales a 60ft cone of magical darkness. A creature with darkvision can't see through this darkness, and nonmagical light can't illuminate it. If the breath overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled. The darkness dissipates after 1 minute.