You hurl the powers of Freezing Death, and Decay.

Level 3: Undead Resilience

You gain Resistance to Cold and Necrotic damage.

Level 3: Undead Spells

The magic of your patron ensures you always have certain spells ready; when you reach a Warlock level specified in the Undead Spells table, you thereafter always have the listed spells prepared.

Undead Spells:

Warlock Level                                               Spells

3                                                            Armor of Agathys, Chill Touch, Ice Knife, Ray of Sickness, Suggestion

5                                                            Phantom Steed, Vampiric Touch

7                                                            Blight, Ice Storm

9                                                            Cone of Cold, Contagion

Level 3: Drain Life

You add the Inflict Wounds spell to your list of Undead Spells. Immediately after you hit a creature with a melee spell attack, you can take a Bonus Action to cast Inflict Wounds on the same creature without expending a spell slot, requiring no Verbal or Somatic components, and it's spell level being equal to your Warlock spell slot level. You gain a number of temporary hit points equal to the damage you deal with that spell. You can cast Inflict Wounds in this way a number of times equal to your Charisma modifier (minimum of 1), and you regain all expended uses when you finish a long rest.

Level 6: Unholy Frenzy

Whenever one of your Undead Spells enables you to make a spell attack, you can attack twice instead of once. You can add you Charisma modifier to one of those two attacks when you reach Warlock level 11, and to both attacks when you reach Warlock level 17.

Level 10: Might of the Lich

Your spells and spell attacks ignore resistance to cold, and necrotic damage, and the radius of your Ice Knife, and Ice Storm spells increases by 5 feet. In Addition, you add the Ray of Frost cantrip to your list of Undead spells.

Level 14: Gifts of the Grave

When you use your Drain Life subclass feature, you can cast either Blight, or Contagion in stead of Inflict Wounds. All other conditions of Drain Life still apply. If you cast the Blight spell in this way, the targeted creature also gains one level of Exhaustion if it fails it's Constitution saving throw against it.

In Addition, you gain Immunity to Exhaustion, Poison damage, and the Poisoned condition. You do not need to breath, eat, or sleep, but you still gain the benefits of short and long rests. If you lose a body part, you can use a magic action to reattach it. If it is destroyed, it will regrow and return to full functionality after finishing a short rest. If damage reduces you to 0 Hit Points, you do not fall Unconscious unless the damage dealt to you is radiant.  Other rules of Dropping to 0 Hit Points stilly apply. If you succeed on your third Death Saving Throw you immediately regain 1 Hit Point. If you roll a 20 on the d20 for a Death Saving Throw, you regain Hit Points equal half of your Hit Point maximum. While you are at 0 hit points, if you are hit by an attack or subject to a spell, that would deal radiant damage to you, you die instantly.