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

Magic Resistance. The devil has advantage on saving throws against spells and other magical effects.

Multiattack. The devil makes three attacks: two with its claws and one with its Bone Whip.

Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 8 (1d8 + 4) slashing damage.

Bone Whip. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 13 (2d8 + 4) piercing damage plus 17 (5d6) poison damage, and the target must succeed on a DC 14 Constitution saving throw or become poisoned for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Spellcasting. Abaddon casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 18):

At will: animate dead (as an action), detect magic, dispel magic, misty step, lightning bolt, mage hand, prestidigitation,circle of death

4/day each: dimension door, invisibility,disintegrate, scrying (as an action)

3/day each: dominate monster, globe of invulnerability, harm,true polymorph,soul cage,plane shift (self only)

Abaddon can take 2 reactions per round but only once per turn.

Fell Rebuke. In response to being hit by an attack, Abaddon utters a fell word, dealing 10 (3d6) necrotic damage to the attacker, and Abaddon teleports, along with any equipment he is wearing or carrying, up to 30 feet to an unoccupied space he can see.

Abaddon has 3 Legendary actions per round. He regains all legendary actions at the start of his turn. 

Melee Attack (1 Action)

Abbadon makes a melee attack with his Bone Whip

Spell Attack ( 2 Actions )

Abaddon makes a spell attack