Celestial knights either spent most of their training days on a celestial plane, learning how to channel radiant energy to use as weapons under the guidance of some sort of holly host or were taken under the wing of a mentor, typically a deva or justiciar, that has seen the potential for goodness and greatness in the trainee. Celestial Knights are forgers of justice destroying evildoers with divine light coated in radiant energy.

Celestial Weapons

3rd-level Celestial Knight feature

As a bonus action, you can channel radiant energy to form a weapon in your hand(s). The weapon can take any form that you choose but uses one of the stat block options below, all damage is radiant damage. Changing from one form to another requires a bonus action. If a saving throw is required for a special property the DC is 8 + your proficiency bonus + your Strength or Dexterity modifier (your choice) : 

Celestial Weapon (Simple)

Celestial Weapon (Martial)

Reach:
5ft.
Damage:
1d8 + Strength Modifier
Properties:
Versatile (1d10), Light
Special Properties:

Celestial Might: Once per turn, on a hit, you can add additional damage equal to your Strength or Dexterity modifier.

Celestial Weapon (Great)

Reach:
10ft.
Damage:
1d12 + Strength Modifier
Properties:
Two-handed, Light
Special Properties:

Celestial Shatter:  On a hit, the target must make a Strength saving throw or be knocked back 5ft.

All weapons count as melee weapons and have the weapon properties defined in the stat block. The weapon disappears when it leaves your hand and needs to be resummoned as a bonus action unless otherwise stated. 

Shield of the Heavens

7th-level Celestial Knight feature

As a bonus action, you can manifest a shield in an empty hand provided you have at least one hand empty. The shield function as a normal shield and provides the typical AC bonus.

Radiant Protection.

As a reaction, when an ally within 30ft of you is attacked you can move the shield to your ally, providing them the AC bonus for that attack, destroying the shield. You will need to use a bonus action to resummon the shield.

Additionally, You can dismiss the shield as a bonus action.

Tempered Celestial Weapons

7th-level Celestial Knight feature

Your celestial weapon now counts as a +1 weapon for attack rolls and damage. This bonus increases to +2 at level 15 and +3 at level 18. Additionally, the weapon dice for the Celestial Weapon forms simple, martial, and great increase by d8, d10 (d12 versatile), and 2d6 respectively.

Armor of Light

10th-level Celestial Knight feature

At the start of your turn, you can wreath yourself in light. The form of this light is determined by you and adds the following benefits:

• You gain temporary hit points equal to your level in this class
• While you have these temporary hit points you have resistance to necrotic and radiant damage
• While you have these hit points you gain +1 to your AC

You can use this feature a number of times equal to your proficiency bonus

Prismatic Armory

15th-level Celestial Knight feature

All of your celestial weapon forms gain the finesse property, and additional special properties detailed below:

Celestial Weapon Simple:

Celestial Nova: When you hit with a ranged attack, you can forgo using celestial burst. The weapon stays embedded into the target. You can have up to your proficiency bonus of weapons embedded into targets at one time. You can expend one of your uses of Celestial Burst to, as an action, you can choose to detonate the embedded weapon(s). The targets each take damage equal to your Strength or Dexterity modifier for each embedded weapon. If the target is a medium creature or smaller it is launched up into the air 5ft per embedded weapon.

Celestial Weapon (Martial):

Celestial Vigor: On a hit, you can add additional damage equal to one die of the weapon plus your Strength or Dexterity modifier. You also heal a number of hit points equal to the damage.  Doing so destroys the celestial weapon and must be resummoned. You can do this a number of times equal to your proficiency bonus.

Celestial Weapon (Great):

Celestial Fury: You can smash the ground in front of you with your Celestial Weapon (Great). Blinding light bursts around you, and each creature of your choice within 10 ft of you must make a Dexterity saving throw or take 3d8 radiant damage and be pushed back 15ft. Creatures take half the damage on a successful save and aren't knocked back. Additionally, the creatures must make a Consitution saving throw or be blinded until the start of your next turn. You can use this feature a number of times equal to your proficiency bonus.

Heavenly Wall.
As a bonus action, you can cause your Shield of the Heavens to hover around you, still giving you the AC bonus. Additionally, once per turn, you can move the shield to an ally you can see, giving them the AC bonus until it is moved. This requires concentration as if concentrating on a spell and last one minute or until concentration is broken. You can do this a number of times equal to your proficiency bonus per long rest.

Citadel of Radiance

18th-level Celestial Knight feature

Your Armor of Light gains the following additional benefits:

While your armor is active you gain immunity to necrotic and radiant damage
You gain a flying speed equal to walking speed, if you lose your armor while flying you fall