Accursed Flames. When the knight deals fire damage, it ignores resistance to fire damage.

Brutal Strikes.  When the knight has advantage on an attack roll with a melee weapon, it can knock the target of the attack prone if the lower of the two d20 rolls would also hit the target.

Douse the Flames of Hope. When a creature deals radiant damage to the knight, the knight has advantage on attacks against that creature until the end of its next turn.

Deathly Choir. Any creature within 30 feet of the knight that is frightened of it and isn't protected by a mind blank spell hears in its mind the screams of the thousands of people whose souls were burned by the dark flame. Any affected creature has its movement speed reduced to 0 and takes 22 (5d8) psychic damage at the start of each of its turns.

Legendary Resistance (3/day). When the knight fails a saving throw, it can choose to succeed instead.

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

Marshal the Unholy. Fiends and undead within 30 feet of the knight have advantage on saving throws against being turned.

Spellcasting. The knight is a 20th level spellcaster. Its spellcasting ability is Charisma (spell save DC 22, +14 to hit with spell attacks). It can cast the following spells:

• Cantrips: green-flame blade, thaumaturgy
• 1st level (4 slots): arms of hadar, command, hellish rebuke, shield, wrathful smite
• 2nd level (3 slots): darkness, hold person, spiritual weapon, suggestion
• 3rd level (3 slots): bestow curse, dispel magic, fireball, spirit shroud (necrotic)
• 4th level (3 slots): blight, shadow of moil, staggering smite
• 5th level (2 slots): circle of power, dominate person, immolation

Unyielding. The knight cannot be moved against its will.

Multiattack. The knight can use its Frightful Presence. It then makes three attacks with its flail or uses its Wild Swing if available.

Darkblood Greatflail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage plus 21 (6d6) fire damage. If the attack is a critical hit, the knight gains temporary hit points equal to the fire damage dealt.

Unholy Bolt. Ranged Spell Attack: +14 to hit, range 120 ft., one target. Hit: 22 (4d10) fire damage plus 22 (4d10) necrotic damage. The target must succeed on a DC 22 Constitution saving throw or has its hit point maximum reduced by an amount equal to the necrotic damage taken.

Frightful Presence. Each creature of the knight’s choice within 120 feet of it and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the knight’s Frightful Presence for the next 24 hours.

Wild Swing (Recharge 5–6). The knight makes an attack with its flail against each creature within 10 feet of it. Each creature hit by an attack is pushed 5 feet away and knocked prone.

Flames of the Underworld (1/day). Any nonmagical flame within 120 feet of the knight is snuffed out and in its place, a black flame appears, which does not spread light, but rather darkness. Furthermore, raging dark flames burst forth from the knight, engulfing and consuming everything around it. Each creature within 30 feet of the knight must make a DC 22 Constitution saving throw, taking 42 (12d6) fire damage and 42 (12d6) necrotic damage on a failure or half as much on a success. In addition, on a failure, a creature's hit point maixmum is reduced by an amount equal to the necrotic damage taken.

The flames continue to shroud the knight for one minute. Until they dissappear, each creature that touches the knight or hits it with an attack while within 10 feet of it takes 9 (2d8) fire damage and 9 (2d8) necrotic damage.

Curse of the Dark Flame. The knight targets a creature within 30 feet of it, which must succeed on a DC 22 Wisdom saving throw or be cursed as the dark flame starts burning away its very vitality. While cursed in this way, it gains a level of exhaustion at the start of each of its turns. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Otherwise, the curse lasts until removed by a remove curse spell or similar magic. If a cursed creature dies, its body is consumed by dark flames and turned into a pile of ash. Once a creature's saving throw is successful or the effect ends for it, it becomes immune to the knight's Curse of the Dark Flame for the next 24 hours.

Evoke Despair.  If the knight can see a creature within 120 feet of it regaining hit points, it can use its reaction to force the creature to make a DC 22 Wisdom saving throw. On a failure, the creature cannot regain any hit points and is frightened until the start of its next turn.

The knight can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The knight regains spent legendary actions at the start of his turn.

Attack.  The knight makes an attack with its flail.

Fire Strider. The knight steps into an open flame that occupies at least a 5 by 5 feet square and reappears within a different flame within 120 feet of the original one. Both flames turn dark and no longer spread light.

Unholy Bolt (Costs 2 Actions). The knight uses its Unholy Bolt.

Magic of the Dark Flame (Costs 2 Actions). The knight casts one of its spells.