Geth, Warlock of the Dark Old One

Innate Spellcasting. The warlock’s innate spellcasting ability is Charisma. It can innately cast the following spells (spell save DC 15), requiring no material components:

At will: alter self, false life, levitate (self only), mage armor (self only), silent image

1/day each: feeblemind, finger of death, plane shift, arcane gate

Spellcasting. The warlock is a 17th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). It regains its expended spell slots when it finishes a short or long rest. It knows the following warlock spells:

Cantrips (at will): eldritch blast, fire bolt, mind sliver, mage hand, minor illusion, sword burst, shocking grasp

1st–5th level (5 5th-level slots): arms of hadar, burning hands, flame strike, hellish rebuke, magic circle, scorching ray, crown of madness, enemies abound, enervation, wall of fire, infernal calling

Dark Old One’s Own Luck (Recharges after a Short or Long Rest). When the warlock makes an ability check or saving throw, it can add a d10 to the roll. It can do this after the roll is made but before any of the roll’s effects occur.

Dark Old One's Alteration. When a warlock makes a pact with the Dark Old One they are magically altered to be similar to Fiends and are given an increase of all spell slots by 1. They can also become a puppet of the Dark Old One making them immune to the conditions: Blinded, Charmed, and Paralyzed.

War Mage.  On a hit, He can cast a cantrip after making a melee weapon attack with no disadvantage from range but, the spell is only half effective depending on the spell. (e.g.: damage is halved, magical effect is halved, or spell save DC is lowered by half)

Multi-attack. He can make two melee attacks or use the War Mage action after making one melee attack 

Mace. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6+3) bludgeoning damage plus 9 (3d6) fire damage.

Hellish Rebuke

Reanimate. When his hit points reach 0 he can comeback to life at half his maximum hit points unless hit with a critical attack then he can only reanimate at a quarter hit points due to the Dark Old One's influence. He can only reanimate 3 times per day before his body is no longer usable. Any lost limbs are not restored upon reanimation, but instead replaced by demonic limbs from the Dark Old One; even if his head is cut off his body can still be puppetted.