As an Arch Angel you represent both your god and the mortals they reside over, you physical appearance partially resembles that of a mortal race. 

You choose a mortal race, you cannot choose a demonic or angelic race, Aasamar and Tiefling for example. 

You take on the stats of that species as an additional. 

As an Angel you are very strong, Intelligent and Charismatic, you gain a bonus in the following traits.

Charisma and Strength increases by 4.

Your Constitution, Intelligence, Wisdom and Dexterity increase by 3.

Even as a Fallen Arch Angel you still retain a few of your natural forms, 

you are ageless and can never die of old age.

A Fallen Arch Angel can be of any alignment due to the various ways that they can fall.

Arch Angels can be of any height, use medium as a guide

Your base walking speed is 40 feet and your base flying speed is 80 feet. Fallen-Angel's have wings that are grey that sway and move around. They can "retract" them at will.

Fallen Arch Angels can see in normal and magical darkness,

2/long rest, see invisible creatures and objects,

2/long rest, detect visual illusions and have an advantage on saving throws against them,

perceive the original form of a shapechanger or a creature that is transformed by magic out to 60 ft.. Furthermore, they can see into the Ethereal Plane.

You have resistance to necrotic and poison damage.

You are immune to Radiant Damage, Charm and Frightening conditions, you can also not be put to Sleep by magic.

You are fluent in all languages.

Your weapon attacks are magical. When you hit with any weapon, the weapon deals an extra 1d8 radiant damage (included in the attack). The Damage increases by 1d8 on 3,6,9,12,15,18.

You have advantage on saving throws against spells and other magical effects.

Your maximum attributes are 30

The Angel will know if it hears a lie. 

The Angel will know a creatures true alignment

The Celestial has the ability to perform some of magic that their True Form could, however at a much more reduced level. 

At will - Detect good and evil, Invisibility (self only)

1/long rest, Commune, Control Weather 

At 3rd level you gain the following -- 3/long rest, Dispel good and evil and 1/long rest, True Resurrection

at 6th level you gain the Blade barrier spell at 1/long rest.

The Angel can, 3 times per long rest, touch another creature and it magically regains 1d8+4 hit points and is freed from any curse, disease, poison, blindness or deafness.

This increases by 1d8 at 3,6,9,12,15,18 and the 20th.

You can teleport yourself and one other creature/object up to 60 feet to an unoccupied space that you can see as a bonus action 2 times per short and long rest.

you emit a magical celestial energy blast, once per short rest each creature of your choice in a 20 foot radius must make a dc14 dex saving throw, taking 1d6 fire damage plus 1d6 radiant damage on a failed save, or half as much on a successful one.

Damage is increased by 1d6 at 3,6,9,12,15 and 18.

A being of good standings to the Angel speaks its true name in a powerful ritual, This true name will include the Angels title. (A secret name of which the player, GM and selected individuals and characters are aware of, the Angel will generally go by a different)

A blinding cone of dark and light energy shoots up into the sky changing whatever weather to a large circular storm centered on the Angel, rain and lighting blast the area for miles around as heaven and earth shakes and the power within awakens, natural dark consumes an area 40 feet from the Angel, 40 -200 feet out loses nearly all natural light, but not total darkness, within 10 feet of the angel the streaming energy of darkness and light parts and a bright stream of light from the heavens reveal that the Fallen Arch Angel has awakened its true power within, emerging from the energy blast in its true form, 6 wings flourish from the back of the angel.

Roll 10d12 for radiant damage, all creatures within 50 feet that are not considered an ally makes a dc 14 dex check,  if failed they take full damage and half that if they succeed, evil creatures 50-150 feet out make a dc15 charisma check, they are effected by fear if they fail.

The Angel has all the powers and abilities of an their chosen Arch-Angel (Law, Order, Wrath, etc) until the end of the transformation window (an hour) or until their HP hits 0, at which point they transform with a blinding explosion back into their original form, causing all creatures within 20 feet to take 2d8 Radiant damage. The Angel loses 75% of their total remaining HP before transformation. 

Dis-advantage on rolls until after 3 long rests.