Demonic Possession

You call upon a wayward demon from the Abyss 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 demon you call upon for the duration. A willing creature can choose to fail the saving throw. If you are familiar with a specific demon, 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 demon 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 Bite or Spit Acid 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).
Bite is a melee weapon attack that deals 1d12 piercing damage on hit and the attacked creature must succeed on a Constitution saving throw or take 4d4 poison and be poisoned for 1 minute. An affected creature can repeat the saving throw at the end of their turns to remove the poisoned condition. Spit Acid is a ranged(30/60 feet) weapon attack and deals 2d8 acid damage on hit, the attacked creature must succeed on a Dexterity saving throw or be covered in acid for 1 minute, while covered in acid, the creature's non-magical gear gains cumulative a -1 penalty to either attack rolls or AC(depending on the gear) at the start of its turn, this reduction is permanent. A creature covered in acid or another creature within 5 feet of it can use its action to make a Dexterity check against your spell save DC, removing the acid on success. Objects that aren't worn or carried automatically fail this saving throw, and they take 5 acid damage at the initiative count of 20.

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 demon 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 demon resistance to poison and bludgeoning, piercing, and slashing damage from non-magical weapons. It also gains immunity to the poisoned condition.
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 demon 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 demon back to the Abyss, 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 Bite and Spit Acid 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 fire and cold damage if you called upon a specific demon. 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 Bite attack has advantage against poisoned creatures, and the Spit Acid attack can target two creatures per attack, as long as they are within 5 feet of each other.
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 demon, it gains a 60-foot aura of decrepit decay. Creatures of your choice that start within the aura must succeed on a Constitution saving throw or be poisoned for 1 minute. While poisoned in this way, the creature cannot regain hit points and takes 4d6 necrotic damage at the start of its turn. The affected creature can repeat the saving throw at the end of their turns. If you so wish, plants and small animals within the aura wild and die and the start of the possessed creature's 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.