Celestial Possession

You call upon a wayward celestial from the Upper Planes to possess a creature within range. This spell cannot be cast on Constructs. The creature must succeed on a Charisma saving throw or be possessed by the celestial you call upon for the duration. A willing creature can choose to fail the saving throw. If you are familiar with a specific celestial, it is willing and able, you can call upon it specifically and it will obey your call, otherwise, the spell fails and the spell slot is wasted. 
You have a telepathic link towards the possessing celestial as long as both of you are on the same plane of existence. The creature acts immediately after you in the initiative order unless you are no longer concentrating on this spell, in which case it rolls its own initiative using the possessed creature's bonuses.

Using a bonus action on your turn, you choose an unoccupied spot to which the possessed creature will move, during this movement you can decide for it to use its action to choose thrice whether it makes a Slam or Radiant Bolt attack using your spell attack modifier for attack and damage rolls, or perform a Healing Touch(these are considered attack options for purposes of multiattack in case of a willing target).
Slam is a melee weapon attack that deals 2d8 bludgeoning and 4d6 radiant damage on hit. Radiant Bolt is a ranged(60/120 feet) weapon attack that deals 4d8 radiant damage on hit. The Healing Touch restores 2d6 hit points to a willing creature within 5 feet of the possessed creature.

Instead of taking the above-described attacks, the possessed creature can also perform any other action that its physical form would allow, it cannot take any non-attack action from its stat block unless DM decides otherwise. If the target of this spell was willing, then the possessing celestial has access to everything in its stat block. For the duration of the spell, the creature gains 40 temporary hit points. And if you called upon a specific celestial resistance to radiant, necrotic, bludgeoning, piercing, and slashing damage. It also gains a flying speed of 60 feet.
The possessed creature(that was not willing) repeats the saving throw at the start of every hour, being unaffected on success until it fails the next saving throw. It must succeed on three saving throws(which don't have to be consecutive) to end this spell. The spell also ends if the possessed creature is reduced to 0 hit points. If you lose concentration, the spell doesn't end, instead, you lose control of the celestial and it can choose what to do for the remaining duration of the spell.
A spell such as Dispel Evil or Good, or alternatively Dispel Magic or Remove Curse cast at the same level as this spell or higher banishes the celestial back to the Upper Planes, completely ending the spell early.

At Higher Levels: If you cast this spell using a spell slot of 7th level or higher, the duration is Concentration, up to 10 days. Additionally, it gains an additional 30 temporary hit points, as well as 30 feet of Truesight and 10-foot aura that grants the same resistances this spell grants to creatures of celestials choice if you called upon a specific celestial. If you cast this spell using a spell slot of 8th level or higher, its duration is Concentration, up to 1 month, and the possessed creature can repeat the saving throw once every day at dawn and the saving throws the possessed creature makes to end the spell have to be consecutive successes to end it. Additionally, the Slam attack has advantage against fiends and undead, Radiant Bolt attack forces the attacked creature to succeed on a Constitution saving throw or be blinded for 1 minute, an affected creature can repeat the saving throw at the end of their turn, also the Healing Touch removes either one disease or condition from its target, which can be either blinded, deafened, charmed, frightened, or poisoned.
If you cast this spell using a spell slot of 9th level, its duration is Concentration, until dispelled, also, the possessed creature can repeat the saving throw once every month. Additionally, if you called upon a specific celestial, it can use its action to burst with a flare of sunlight from its body. Creatures of its choice within 120 feet of it must make a Constitution saving throw. Creatures that fail on the saving throw take 8d8 radiant damage and are bathed in sunlight for 10 minutes. Affected creatures are blinded, shed 5 feet of bright light and 5 feet of dim light(this light is sunlight), and take 2d8 radiant damage at the start of their turn, they can repeat the saving throw at the end of their turn to end the effect early. Creatures that succeed on the initial saving throw take half damage and are not bathed in sunlight. All magical darkness within the area is dispelled and objects within the area that are not worn or carried are bathed in sunlight but do not take any radiant damage. At the start of the possessed creature's turn, roll a d6, on a result of 5-6, this ability recharges. 
If the possessed creature is reduced to 0 hit points, it doesn't fall unconscious, instead, it makes a concentration check whenever it takes damage as if you were making it, only ending the spell after it fails.

Regardless of how often the creature gets to make the saving throw against this spell, if it succeeds, it must make the next saving throw at the start of its next turn.