Corrupted Knight 'O the Mountain King

Oath of Bloodshed Paladin. If a paladin rolls a Wisdom (Perception) or Intelligence (Investigation) check of 15 or higher, they discover that this creature was once a level 14 paladin under the Oathbreaker subclass, but has undertaken a new oath under the rule of the demon who has corrupted this unfortunate paladin. This paladin can choose to undertake the Oath of Bloodshed (see my homebrew creations) as their new oath after defeating the paladin in battle if they are currently an Oathbreaker paladin of any level.

Champion's Readiness. Whenever this creature rolls for initiative, it's score cannot be lower than a 19 and cannot be surprised.

Flame Propulsion. Whenever this creature uses it's flying speed to move, it leaves behind a trail of fire that lights any flammable tiles within it's grasp ablaze.

Corrupted. Whenever this creature drops to 0 hit points, it is no longer corrupt and instead returns to 17 Hit Points. It's alignment changes to Neutral Good and assists the creature who dealt the final blow to uncorrupt them to the best of their ability. Whenever this creature is uncorrupted, it loses it's legendary actions. After the battle is over, this creature swiftly dies.

Spellcasting. The Corrupted Knight 'O the Mountain King is a 5th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 21, +16 to hit with spell attacks). The Corrupted Knight 'O the Mountain King has following spells prepared:
Cantrips (at will): green-flame blade, eldritch blast
1st level (6 slots): hellish rebuke, inflict wounds, vicious mockery, create or destroy water
2nd level (5 slots): branding smite, heat metal, misty step, scorching ray
3rd level (5 slots): spirit shroud, fireball, summon lesser demon
4th level (4 slots): summon greater demon, fire shield
5th level (3 slots): destructive wave, flame strike

Multiattack. This creature can make 3 melee attacks, 2 spell attacks, or 2 ranged attacks as an action.

Greatsword of Heroes. Greatsword Attack: +18 to hit, reach 5 ft., 1 target. Hit: 13 (2d6 + 7) slashing damage, 3 (1d4) fire damage. 

Longbow. Longbow Attack: +11 to hit, range 150/600 ft., 1 target. Hit: 6 (1d8 + 1) piercing damage.

Shove. This creature can take the 'Shove' action as a bonus action.

Anti-Magi Rite. As a reaction to being targeted for a spell that is 3rd level or lower, this creature can negate the spell entirely. They can do this only 3 times per battle.

Counterattack. As a reaction to being missed by a melee attack within 5ft, this creature can make one melee attack against the creature who declared the attack.

The Corrupted Knight 'O the Mountain King can perform 5 legendary actions per day.

Call of Demonic Presence (Costs 2 Actions). Corrupted Knight 'O the Mountain King calls their inner demon to the battlefield, summoning a Nalfeshnee into play as a familiar at the same initiative as Corrupted Knight 'O the Mountain King. This demon shares the health pool of Corrupted Knight 'O the Mountain King and can make attacks as if it were a normal demon. This demon familiar leaves after 1 minute has passed.

Grand Sword of Hellish Fury (Costs 1 Action). Corrupted Knight 'O the Mountain King charges their sword with their black demonic ichorous flames before delivering an attack with their greatsword. If the attack lands, the creature on the recieving end of this damage must make a Constitution save (Spell save DC 21) or become engulfed in flames, taking 2d6 necrotic and 1d12 fire damage at the end of their turns until doused. Creatures affected by this fire can douse themselves as a bonus action in combat, repeating the save until they succeed. For each failed save, the Constitution save DC lowers by 1 point.

Demonic Immolation (Costs 1 Action, lasts 5 turns, Concentration). Corrupted Knight 'O the Mountain King can forgo their action to light thyself ablaze with black demonic ichorous flames. While the flames are active, all enemy creatures who end or start their turn within 15ft of Corrupted Knight 'O the Mountain King take 2d6 necrotic and 1d12 fire damage. This legendary action can be negated by a 7th level 'Counterspell.' If this legendary action is negated this way, it cannot be used for 3 turns.