Warlocks who follow a demonic patron host demons, giving them a physical body in exchange for partial control and greater power. This job is sacred for Templar warlocks, who are inducted into these ranks in a tightly-kept secret ceremony called The Opening. This eldritch ritual not only opens the host's body to the demon, but also strengthens the warlock's will, aiding them in fighting the demon's chaos. These warlocks gain some of the demon's power, but also have the chance to be destroyed by the demon. Surprisingly, almost none of these warlocks survive. 

Possession

As a Bonus Action, you may enter or transform into your demonic form, a hybrid form, or your regular form. Special features are granted for the hybrid and demonic forms, described below. 

When entering your demonic form, you must succeed on a Wisdom saving throw (DC 13 + the amount of times you've made this save since your last long rest) or lose control of your body as your demon takes control of it. While the demon controls your body, on your turn, it makes melee attacks or casts randomly determined spells against a random creature it can see. Outside of combat, the demon behaves as if it has gained the 96-100 Long-Term Madness feature ("I've discovered that I really enjoy killing people.")

This possession can be cured by Greater Restoration, Remove Curse, Protection from Evil and Good or similar magic; however, you retain this feature and the demon remains in your body. 

When you transform into your hybrid form, you must succeed on a Charisma saving throw (DC 13 + the amount of times you've made this save since your last long rest), or not transform into a hybrid and gain a form of Short-Term Madness (DM's choice or roll on the Madness tables in the DMG). 

Demonic Blessings

Choose two features from the features below. You may use these features a number equal to your Proficiency Bonus as a Bonus Action, and you regain all expended uses once you finish a Long Rest. Any saving throws required for the following features equals 8 + your proficiency bonus + your Wisdom or Charisma modifier (choose one). You also may amplify these options, to gain the bonuses described below. To amplify an option, you must take necrotic damage equal to 1d8 + your spellcasting ability modifier. This damage cannot be prevented by Resistances, Immunities, or spells. You gain another feature at level 10 and another at level 14. You may change one of your options at the end of each of your Long Rests, but you must fulfill the level prerequisite for all invocations. 

Demonic Strike (Hybrid or Regular Form Only, Level 6). Your patron invokes your attacks with unholy power. For 1 minute, your weapon and spell attacks deal an extra damage die on a hit, additionally, a creature dealt damage by your attacks must also succeed on a Wisdom saving throw or become Frightened of you until the end of their next turn. 

Amplify. This rite lasts for 10 minutes. 

Unholy Aura (Demonic or Hybrid Form Only). Shadowy tendrils of smoke wreath out from you as your patron surrounds you with necrotic energy. At the start of each of your turns for 1 minute, choose a creature within 30 ft. of you. That creature must succeed on a Wisdom saving throw or become Frightened of you for 1 minute. At the start of each of its turns, the target can repeat the saving throw, ending the condition on a success. 

Amplify. You also gain a +2 bonus to Armor Class for this duration. 

Necrotic Blade (Hybrid Form Only). A blade of necrotic force appears, hovering, in a space within 10 ft. of you. As a Bonus Action on subsequent turns, you may move the weapon up to 30 ft. and make a melee weapon attack (using your spell attack modifier) with it against a creature. On a hit, the creature takes 2d8 + your spellcasting ability modifier necrotic damage. 

Desecrate (Any Form, Level 10). You channel demonic energy through you, targeting a creature within 60 ft. of you. Choose an ability. For 1 minute, that creature has disadvantage on attack rolls, ability checks, and saving throws using that ability.

Amplify. Additionally, for the duration, that creature cannot regain hit points, and any spells or attacks that creature makes that would deal radiant damage automatically miss. 

Monstrous Presence (Demonic Form Only, Level 14). An emanation of necrotic energy forms around you. For 10 minutes or as long as you do not have the Incapacitated condition, any creature that starts its turn within 15 ft. of you must succeed on a Wisdom saving throw or become Frightened of you until the end of your next turn. 

Amplify. Whenever a creature fails its saving throw, they take psychic damage equal to the damage dealt to you by this feature. 

Flight of the Puppet (Any Form, 10th level). Choose one creature within 30 ft. of you. That creature must succeed on a Dexterity saving throw or be lifted into the air as your demonic energy controls it. For 1 minute, you can take a Bonus Action on each of your turns to command the creature to move up to 30 ft. in any direction. At the end of each of its turns, the creature may make a Dexterity saving throw, ending the effect on them on a success. 

Amplify (14th level). You may target an additional creature within 10 ft. of the first target. 

Poisonous Form (Any Form, 6th Level). For the next minute, whenever you hit a creature with an attack roll, you may deal an extra 2d6 poison damage to the creature as tendrils of green smoke wreath through your blade. 

Necrotic Resistance

You gain Resistance to Poison and Necrotic damage, and you have advantage on saving throws against the Frightened, Charmed, and Paralyzed conditions. 

Vengeful Warrior

Whenever you are hit by an attack roll, your spell attack rolls have advantage against the attacker for 1 round. Additionally, the creature has disadvantage on any spell saving throws against your spells for the same duration. 

Possessor

As a Magic action, you may target a creature that you see within 60 ft. of you. That creature must succeed on a Wisdom saving throw or become Charmed as your demon takes control of its body. On each of your turns as a Bonus Action, you may command it to do any non-suicidal action. Every hour, whenever it takes damage from you, or whenever it is 1 or more miles away from you, it may make a Wisdom saving throw, ending the effect on itself on a success. You may have only one creature under this effect at a time, and you can use it once a Long Rest.