Forbidden Knowledge

While most warlocks are mortals who have sought power from otherworldly beings, there are those who instead delve into practices long since made taboo. Whether you found these forbidden paths buried in secrecy by arcanists and interplanar powers, or you simply began cutting corners in either technique or morality in your own reckless magical studies, you are more independent than most warlocks. Unfortunately, this does little to lower the price of playing god. Regardless of the loopholes one believes they have found, the nature of power is to always come at significant cost. 

(Optional: You can ask your DM if they will allow this kind of warlock to use intelligence as their casting attribute instead of charisma.)

Expanded Spell List

Forbidden Knowledge lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Fiend Expanded Spells

Spell Level	Spells
1st	magic missile, longstrider
2nd	alter self, enhance ability
3rd	animate dead, call lightning
4th	death ward, vitriolic sphere
5th	planar binding, teleportation circle

The Fruits of Hubris

Once per long rest, you can choose to activate one of the four effects below as an action. Whenever you're knocked unconscious by falling to 0 hit points or whenever you gain a point of exhaustion, your volatile magic activates one of the effects at random. (roll 1d4 to determine the effect)

1. Neurosis: Forbidden knowledge begins to echo obsessively in your mind. You can cast dissonant whispers once without spending a spell slot at the level of your proficiency bonus minus 1. If this spell is not used, it is cast on you automatically at the end of the next rest you take using your spell save DC.

2. Depression: Your body absorbs the life-energy in the area as a defense mechanism. Every creature within a 10-foot radius circle around your body makes a dexterity saving throw against your spell save DC or take 2d6 necrotic damage.

3. Mania: Your body refuses to be slowed by it's natural limits. You heal hp equal to your warlock level at the end of your turn, but gain a random deformity determined by the DM until you finish a long rest. Deformities might include additional eyes, mouths, horns, fingers, or warped proportions.

4. Psychosis: The voice of something beyond this world is speaking to your mind, intrigued by your strange condition. You are treated as if you were under the effects of a contact other plane spell, except you can only ask one question, the damage for failing the save is equal to your character level, and the insanity's only effect is to prevent you from speaking sensibly for ten minutes. The entity contacted is typically your patron or the source of your forbidden knowledge, but can be random or simply chosen by the DM.

(Optional: You may add this feature to any subclass of warlock as an invocation, except the effects are always random.)

Blasphemous Experiments

Whenever you finish a short or long rest, you can choose one necromancy spell from any class spell list to temporarily learn as a warlock spell until your next short or long rest. Since warlocks do not normally have access to spells like animate dead or create undead, you can only control a number of undead raised by warlock spells equal to your warlock level.

Cursed With Genius

Starting at 6th level, you can spend a hit dice as a bonus action to roll it and add it's result to a spell attack you make or to the spell save DC on one creature's save against a spell you cast. This can be activated after seeing the initial roll but before any of the roll's effects occur.

Spite the Powers

Starting at 10th level, You can use your reaction to end any temporary condition or disease on a creature you can see within 60 feet (Including yourself,) besides incapacitation or exhaustion, or prevent petrification or disintegration if this ability is used at the moment one of those effects begins to take hold. You gain a point of exhaustion each time this ability is used, activating your Fruits of Hubris feature. 

You recover from two levels of exhaustion each long rest instead of one.

(A temporary condition is any condition that allows automatic repeating saves to end, or has a finite duration.)

Toy with Nature

Starting at 14th level, you can cast a necromancy spell you know at 8th level once per long rest without spending a spell slot.