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

magic weapons. the abyssal champions weapon attacks are magical

Marshal demons. Unless the abyssal champion is incapacitated, it and demonic creatures of its choice within 60 feet of it have advantage on saving throws

Spellcasting. The abyssal champion is a 12th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It has the following  spells prepared:

1st level (4 slots): command, compelled duel, wrathful smite

2nd level (3 slots): hold person, 

3rd level (3 slots): dispel magic, Hunger of Hadar

4th level (3 slots): banishment,vitriolic sphere

5th level (2 slots): destructive wave (necrotic),cloudkill

Multiattack. The abyssal champion makes two abyssal longsword attacks.

 abyssal Longsword. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 19 (3d8 + 5) force damage, or 22 (3d10 + 5) force damage if used with two hands, plus 18 (4d8) poison damage.

corrosive Orb (1/Day). The abyssal champion hurls a magical ball of bubbling acid that explodes at a point it can see within 120 feet of it. Each creature in a 20-foot-radius sphere centred on that point must make a DC 16 constitution saving throw. The sphere spreads around corners. A creature takes 28 (8d6) acid damage and 14 (4d6) necrotic damage on a failed save, or half as much damage on a successful one.

Parry. The abyssal champion adds 4 to its AC against one melee attack that would hit it. To do so, the abyssal champion must see the attacker and be wielding a melee weapon.