At 1st level, you manifest an aspect of your patron’s dreadful power. As a bonus action, you transform for 1 minute. You gain the following benefits while transformed:

• You gain temporary hit points equal to 1d10 + your warlock level.
  

• Once during each of your turns, when you hit a creature with an attack, you can force it to make a Wisdom saving throw, and if the
  saving throw fails, the target is frightened of you until the end of your next turn.

• You are immune to the frightened condition.
  

You can transform a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

The appearance of your Form of Dread reflects some aspect of your patron.

http://dnd5e.wikidot.com/warlock:undead

At 6th level, your patron’s powers have a profound effect on your body and magic. You don’t need to eat, drink, or breathe.

In addition, once during each of your turns, when you hit a creature with an attack and roll damage against the creature, you can replace the damage type with necrotic damage. While you are using your Form of Dread, you can roll one additional damage die when determining the necrotic damage the target takes.

http://dnd5e.wikidot.com/warlock:undead

At 10th level, Your connection to undeath and necrotic energy now saturates your body. You have resistance to necrotic damage. If you are
transformed using your Form of Dread, you instead become immune to necrotic damage.

In addition, when you are reduced to 0 hit points, you can use your reaction to drop to 1 hit point instead and cause your body to erupt with deathly energy. Each creature of your choice that is within 30 feet of you takes necrotic damage equal to 2d10 + your warlock level. You then gain 1 level of exhaustion. Once you use this reaction, you can’t do so again until you finish 1d4 long rests.

http://dnd5e.wikidot.com/warlock:undead