Hell Knight

(This is subclass is not meant to be used in serious campaigns. It is far more powerful than anything in the core rules.)

Some Warlocks make a deal with Cthulhu. Cthulhu makes deals with you.

Level 3: Blessing of the Hell Lord

When you reduce an enemy to 0-5 Hit Points, the target instantly dies and you gain Temporary Hit Points equal to your Charisma modifier plus your Warlock level (minimum of 1 Temporary Hit Point). You also gain this benefit if someone else reduces an enemy within 10 feet of you to 0-5 Hit Points. Temporary HP from this feature stacks.

Level 3: Dark Knight of Agony

You have proficiency in all weapons, all armor, and shields. You can ignore the strength requirement of heavy armor and your weapon attacks use Charisma instead of Strength or Dexterity for attack and damage rolls.

Additionally, you deal 1d6 extra necrotic or psychic damage (your choice) whenever you hit a creature with a weapon attack.

Level 6: Hellish Ward of Entropy and Destructive Chaos

When you are hit with a weapon attack, you can use your reaction to cast a hellish ward, adding +5 to your armor class until the beginning of your next turn and possibly causing the triggering attack to miss. Whether the triggering attack hit or missed, the creature who targeted you is enveloped with fire and entropic energy. The creature must make a Dexterity saving throw, taking 2d10 fire damage and 4d10 + Warlock level necrotic damage on a failed save and half as much on a successful one. 

Level 10: Dark Soul of Resilience

When you are targeted by a spell of 6th level or lower, you can choose to ignore the effects.

Level 14: Complete and Utter Darkness, Agony, Vengeance, Emptiness, Destructive Force, and Loathing

When you hit a creature with a weapon attack, you may expend a Pact Slot to instantly banish that creature to the 9th layer of the Abyss.

Additionally, if you die, you are instead banished to the 9th layer of the Abyss.