Devil's Contract

Otherwise known as a Faustian Pact within the communities of devils, the caster is capable of summoning two types of contracts. Known as Pact certain, and Pact Insidious.

The Pact Certain is a contract that the caster might prefer more, because it was fast. The signer immediately handed over his or her soul to the caster on death for some services. Barring proving that the pact was signed under some form of duress like threat of torture, these pacts' terms were impossible to adjudicate

The Pact Insidious was a bit different. Here, a signer and the caster agreed upon exchanging services. Namely, the caster provided the signer with something and the mortal had to act in some manner that the caster wanted to. It was possible to make multiple follow-up contracts that gave the receiving mortals more benefits in return of additonal behaviors it had to follow. Failing to act in such manner meant that the mortal lost the contract's benefits. Agreeing to such a Pact Insidious in itself was not an act that shackled one's soul to the ownership of the caster, but acting in ways that were specified within the contract or contracts meant that one acted in ways that put him or her ever closer to reach Baator or the caster on death. Discussing the issue with others was impossible for mortals because a clause that was in every Pact Insidious was that forbade the option of discussing the contract's content with other people.

For warlocks of the fiend pact
Once a the signer of the contract has perished (Preferably Pact Certain), the warlock will be allowed to keep the soul in their posession for the time being
(Roll a d3 to see if the patron will accept this, 1: yes 2: no 3: for an amount of days decided by the dm. roll for each soul obtained) 
and if the warlock is allowed to keep the soul in their posession. They may choose one of three options

1. Devour the soul
The warlock will consume the soul, gaining temporary hp equal to half of what the consumed soul had during its life. But will also risk the chance of slowly turning into a nabassu
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Roll a d7

1. Fly as the truly damned
The warlock (in d5 days) will grow a pair of black leathery wings, which are capable of flight

2. Charred Punishment
The skin of the warlock will slowly burn off from the great sin even demons despise, revealing new, black skin from underneath the old

3. Hands of an outcast
The warlock's hands and feet will mutate, bones extending their fingers and toes. The tips forming into claws, and the skin on them turning black

4.Eternal Hunger
The caster grows a hunger for more souls, forced to seek prey to devour.
(If afflicted by N.4, the warlock must roll a d100 for each soul they obtain. 50 or lower, the soul is eaten. 51-80 the soul is passed to the patron of the warlock, and on 81-100, the soul slips off from the grip of the warlock, escaping into the afterlife)

5. Hated existence
The caster emits the aura of a nabassu, drawing the anger and hatred of all fiend races. In combat the fiends will focus on solely the warlock, wanting to erase the devourer off the world.
Out of combat, roll all charisma based rolls with disadvantage.
And even if out of combat, the warlock will be targetted by any demons that see it, as they despise the soul eaters

6. Jaw of the eater
The warlock's jaw will transform into that of a nabassu, growing sharp teeth capable of biting attacks

7. Mark of damnation
The warlock will grow a pair of horns, and their eyes will become bright yellow. And will be unable to see as easily in the light
(When in light, your vision will be 10ft, this will be removed once completely turned into the hated demon)

Once the warlock has all of these, their sin will turn them completely into a nabassu
See nabassu from the monster manual, and turn your alignment into Chaotic Evil
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2.Give the soul to your master
The warlock will by magical means lead the soul to bator, or the abyss. To whichever realm their master resides in

3.Let it go
The soul will be released into the afterlife 

If you die with this spell in your arsenal
You will be reborn in the circle of hell that your patron inhabits
If you die and you have eaten a soul
You are reborn in the abyss as a nabassu
For now
Paladins get the same choices as warlocks, i do not have the mental health to write more