Battle Magic. When the Darksong Knight casts a spell as an action, it can make an attack with its blade of radiance as a bonus action.

Blessed Hunter. The Darksong Knight has advantage on Wisdom (Perception) and Wisdom (Survival) checks to track and to find fiends and undead. In addition, the Darksong Knight scores a critical hit on a roll of 19 or 20 on the d20 when making an attack with its blade of radiance against a fiend or undead.

Elusive Footwork. Opportunity attacks against the Darksong Knight are made with disadvantage.

Fey Ancestry. The Darksong Knight has advantage on saving throws against being charmed, and magic can’t put the Darksong Knight to sleep.

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

Spellcasting. The Darksong Knight is a 20th level spellcaster. Its spellcasting ability is Charisma (spell save DC 19, +11 to hit with spell attacks). The Darksong Knight knows the following spells:

• Cantrips (at-will): guidance, dancing lights
• 1st level (4 slots): bless, protection from evil and good, searing smite, zephyr strike
• 2nd level (3 slots): misty step, moonbeam, spiritual weapon, zone of truth
• 3rd level (3 slots): blinding smite, crusader's mantle, dispel magic, spirit shroud
• 4th level (3 slots): banishment, fire shield, freedom of movement
• 5th level (2 slots); banishing smite, circle of power, steel wind strike

Multiattack. The Darksong Knight makes four attacks with its blade of radiance. It can use its Feint in place of one of these attacks.

Blade of Radiance. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 9 (1d8+5) slashing damage if used with one hand or 10 (1d10+5) slashing damage if used with both hands, plus 13 (3d8) radiant damage.

Feint. The Darksong Knight targets a creature within 5 feet of it and performs a feint to deceive it. The Darksong Knight makes a Charisma (Deception) check contested by the target's Wisdom (Insight) check. If the Darksong Knight succeeds, the Darksong Knight's next attack against that creature has advantage. If that attack hits, the creature takes an additional 13 (3d8) radiant damage and if it was concentrating, it has disadvantage on its Constitution saving throw to maintain concentration. 
In addition, the creature must make a DC 19 Dexterity saving throw or be knocked prone.

Crescent Moon Strike (Recharge 5–6). The Darksong Knight swings its sword, sending blades of silvery light in the shape of crescent moons forward within a 60-foot cone, dispelling any magical darkness in that area. Each creature of the Darksong Knight's choice within that area must make a DC 19 Dexterity saving throw, taking 54 (12d8) radiant damage on a failure or half as much on a success. In addition, on a failure, a creature is blinded until the end of its next turn and knocked prone.

Eilistraee's Branding Light. The Darksong Knight sings and calls for Eilistraee's light to mark its enemies in battle. Each fiend or undead within 30 feet of the Darksong Knight must make a DC 19 WIsdom saving throw or have its movement speed halved and become outlined by silvery light for one minute. Any attack roll against an affected creature has advantage, and affected creatures cannot benefit from being invisible.
A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can't assume a different form for the duration.

Swift Riposte. The Darksong Knight adds its Charisma modifier to its AC against one melee weapon attack that would hit it. To do so, the Darksong Knight must see the attacker and be wielding its blade of radiance. If this causes the attack to miss, the Darksong Knight can then make an attack with its blade of radiance against the attacker as part of its reaction.

Moonlight Flare. If the Darksong Knight can see another creature within 30 feet of it making an attack, it can use its reaction to force that creature to make a DC 19 Constitution saving throw. On a failure, it is blinded until the end of the turn.