As a bonus action, you can change the damaged property of the blade to one of the damage types in the chart. 

You are also able to wield heavy weapons one-handed and ignore the heavy property. Ie dual wielding heavy weapons

At level 10 you can choose two damage types at once

The host heals everytime it attacks level x hitdie

Hitting all attacks creates infernal chains that drag creatures closer to the darkin. 

30ft cone going outwards 

Wings Sprout off your back letting you fly 40ft

asi

Celestialites Skin = unarmored defense 

Bonus unarmored defence Dex + Bonus unarmored defence con

Gain Magical Darkvision 200ft and True Sight 120ft 

gain expertise at level 15 on all skill and saving throws 

The Host gains  Immunity  to poison and diseases 

at level 15

at level 20, the Celestialite blade can move 20 ft away from the host. 

The blade can only go sentient based on the host Str modifier

Bonus action to activate the sentient blade

It acts after your turn. 

Actions it can do. 

Multi attack : 3 cleave attacks. 

Cleave: Uses the base stat of the Darkin Blade

Divine Chains: The blade summons infernal chains to capture targets. It can capture x (str modifier + proficiency bonus + con modifier) amount of creatures. 
The Creature must make a str saving throw DC to escape the chains is 8 + str modifier + proficiency bonus + con modifier. The range of the chains is 30ft for multiple targets however if it's just one target the range is  60ft 
and the DC to escape is (16 str modifier + proficiency bonus + con modifier )

healing that exceeds your maximum hit points is converted into temporary hit points, as the healing meant for war fortifies rather than mends. 

Every overheal stacks into temp hp and overwrites other temp hp source

If you are at full hit points, all healing you would receive is automatically converted into temporary hit points.

As a bonus action, you can transfer a chosen amount of your temporary hit points to other creatures within range. If the targeted creature is not at full hit points, they are healed by that amount. They gain the temporary hit points if they are at full hit points instead.

Celestialite Shape : change size between medium to large at will 

Reaching level 15 

The Divine Celestial Magic in your body grows stronger,  whenever you make an Attack Roll that lets you add your proficiency bonus, you can treat a d20 roll of 9 or lower as a 10.