Brave. The knight has advantage on saving throws against being frightened.

Multiattack. The knight makes two melee attacks.

Greatsword. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage.

Whip. Melee Weapon Attack: +5 to hit, reach 10 ft., 1 target. Hit: 6 (1d4 + 4) slashing damage, plus 3 (1d6) fire damage.

Charm. One humanoid the fiend can see within 30 feet of it must succeed on a DC 15 Wisdom saving throw or be magically charmed for 1 day. The charmed target obeys the fiend's verbal or telepathic commands. If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success. If the target successfully saves against the effect, or if the effect on it ends, the target is immune to this fiend's Charm for the next 24 hours.

The fiend can have only one target charmed at a time. If it charms another, the effect on the previous target ends.

Hell's Blessing. As an action, the fiend can touch a creature within 10 ft. of it. The creature must succeed on a DC 15 Charisma saving throw or take 3d10 necrotic damage and be charmed by the fiend. While charmed, the creature slowly changes it's alignment. every 1d10 hours, the creature changes it's alignment, changing in accordance to the table at the bottom. It changes from Chaotic to Neutral to Lawful first, before it changes slowly to evil. The effect can be ended by a Remove Curse, Greater Restoration, or similar magic of 5th level or higher. This trait can only be used once every two days.

Chaotic  =>  Neutral  =>  Lawful

Good  => Neutral  =>  Evil

Telepathic Bond. The fiend ignores the range restriction on its telepathy when communicating with a creature it has charmed. The two don't even need to be on the same plane of existence.