Zariel makes pacts with warlocks to bolster her ranks against the demon hordes after their death. In life, she grants them power and ability so that they can grow more powerful. She allows them to live rich full lives of their own making. However, the cost for their power is their souls. On their death, Zariel claims them and reforms them into a devil if her choosing. They retain all their power they amassed from life and are conscripted into her army to battle in the abyss, or whatever else she decides. Once the pact is complete, the warlock loses the ability to deny Zariel anything she should so choose.

Class spells

Class spells as required

Warlock priest of Zariel

At level 1, Zariel gives her priests the ability to choose and cast spells from the transmutation school of cleric spells as if they were a priest of the same level in an attempt to divinely drive their quest to achieve power. 

Infernal Tongues

Starting at 1st level, you gain internal tongues from Zariel. You can choose a language and instantly know this language for one hour will enough to read, write, or speak it. Once used you have to at least complete a short rest to be able to use it again. 

Zariel's Favored

At 6th level, you begin to stand out from the normal weak mortals to Zariel. She begins to look upon you with favor. Damage you deal from any source or spell is unusually effective when used against celestial or demonic creatures. You double all dice and modifiers against celestials and triple them against demons. This enemies of the nine hells will soon learn to respect your power.

Devil's Nature

Starting at 10th level, your body undergoes its first change as Zariel's corruption of you soul completes. If you were not lawful evil, you become lawful evil.  You gain the devil archetype in addition to your normal race. You become immune to non-magical fire. You become resistant to all other types of fires for half damage.  You begin to become a devil. 

Chosen of Zariel

Your transformation into a devil is complete at level 14.  You gain full telepathy and telekinetic abilities. You are able to move things you could normally physically carry with your mind from up to a distance of 15 plus your INT modifier number of feet away. You also gain devilish wings on your back that grant you the ability to fly at your movement speed but give away your devil nature. The wings can not be hidden by anything short of magical means and impose a disadvantage on all persuasion, deception, or performance checks verses any being of good alignment.