Devilish Possession

You call upon a wayward devil from the Nine Hells 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 devil you call upon for the duration. A willing creature can choose to fail the saving throw. If you are familiar with a specific devil, 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 devil 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 twice whether it makes a Claw or Hurl Flame attack using your spell attack modifier for attack and damage rolls(these are considered attack options for purposes of multiattack in case of a willing target).
Claw is a melee weapon attack that deals 2d8 slashing damage on hit and the attacked creature must succeed on a Charisma saving throw or have disadvantage on the next attack roll, ability check, or saving throw it makes. Hurl Flame is a ranged(30/60 feet) weapon attack that deals 2d8 fire damage on hit, creatures within 5 feet of the attacked creature must succeed on a Dexterity saving throw or be lit aflame for 1 minute, while lit aflame, the creature takes 2d8 fire damage at the end of its turn. A creature lit aflame or another creature within 5 feet of it can use its action to make a Dexterity check against your spell save DC, dousing the flame on success. Objects within the area that aren't worn or carried automatically fail this saving throw.

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 devil 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 devil resistance to fire, cold, and bludgeoning, piercing, and slashing damage from non-magical weapons.
The possessed creature(that was not willing) repeats the saving throw at the start of its turn, 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 possessing devil 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 devil back to the Nine Hells, completely ending the spell early.

At Higher Levels: If you cast this spell using a spell slot of 5th level or higher, the duration is Concentration, up to 8 hours. If you cast this spell using a spell slot of 6th level or higher, the Duration is Concentration, up to 1 day,  also, the possessed creature can repeat the saving throw once every hour and you can choose three times between the claw and hurl flame attacks.
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 resistance to lightning and poison damage if you called upon a specific devil. 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, creatures that fail the saving throw from the Claw attack have disadvantage on their attack rolls, ability checks, and saving throws until the end of their next turn, and the Hurl Flame attack deals an additional 2d8 fire damage and leaves a burning area for 1 minute. Creatures that start their turn in the area must succeed on a Dexterity saving throw or be lit aflame as per the description of the attack.
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 devil, once per day, it can perform a 1-hour ritual to summon a Chained Devil under its command for 1 day. As long as you are concentrating on this spell, the Chained Devil is under your command, otherwise, the devil commands it. It rolls its own initiative during combat, you have a telepathic link to it as long as it's within 1 mile of you, and you choose what it does during its turn.
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.