EDIT: This warlock patron has been mostly replaced in my games with the Undying patron found in the Sword Coast Adventurer's Guide. Though less vampire-themed, that pact option seems close enough to this concept to nullify it. If you're interested in a vampire-themed spellcaster, check out the Vampire Bloodline sorcerous origin.
You have made a pact with a powerful undead being. Since before history began, vampires, liches, death knights, and their god-like masters have struck bargains with mortal servitors, who manage their earthly affairs and keep meddlers from prying. In exchange for complete devotion you learn the necromantic arts, playing the dangerous odds between life and death.
Undead creatures powerful enough to forge a pact include Volgothyde, the god of vampirism; Iarmov the Maimed, whose undeath continues as a disembodied spirit; Sirivus, called the Awakener; Halkhunti the Forgotten Pharoah; and Tormadnon, the lich who dwells at the heart of the Vitollos.
EXPANDED SPELL LIST
The Undead 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.
Spell Level Spells
1st false life, sleep
2nd gentle repose, invisibility
3rd animate dead, feign death
4th arcane eye, greater invisibility
5th dominate person, scrying
PACT OF THE CHAIN OPTIONS
When you choose the Pact of the Chain, you gain access to the following familiar forms: giant fire beetle, scorpion, swarm of bats, swarm of insects, and swarm of rats.
Starting at 1st level, the first time each day that damage reduces you to 0 hit points, you can make a Constitution saving throw with a DC of 5 + the damage taken. On a success, you drop to 1 hit point instead. Once you use this feature, or if you take radiant damage, you can’t use it again until you take a short or long rest.
At 6th level, your body can access unnatural levels of performance through your undead patron. When you make a Strength, Dexterity, or Constitution saving throw, add 1d10 + your Charisma modifier. You can do so after seeing the initial roll, but before any of the roll’s effects occur.
Once you use this feature, you can’t use it again until you finish a short or long rest.
FLESH OF MY FLESH
At 10th level, your body becomes accustomed to dealing with the undead and forces that animate them. You gain resistance to necrotic damage, and your hit point maximum can’t be reduced.
CHILDREN OF THE NIGHT
Starting at 14th level, you can use an action to magically call 1d4 swarms of bats or rats, provided that the sun isn’t up. While outdoors, you can call 3d6 wolves instead. The called creatures arrive in 1d4 rounds, and will remain at your side. If an enemy attacks you, the called creatures move toward it and attack. The creatures leave after 10 minutes, or when you dismiss them with a bonus action.